I don't play a lot of 3v3 and 4v4, but I would imagine the later tech transition is in part due to the much larger army you can bring to bare in early stages (having 4 people to draw on). I'm not sure how to help with that, but I can give you the 1v1:
Tech transitions should be part of your overall strategy. If your strategy is: Speedling into Infestory/Baneling into Ultralisk (the TLO ZvT), then you have clearly defined goals as to when to tech. You know that you'll have early harassment with speedlings, you switch to Baneling/Infestor as he starts to get Medivacs for his MMM, you'll upgrade melee damage, and once he gets his factory up and producing you'll move to Ultralisks.
Day9 once said "every [good] strategy has a beginning, a middle and an end." This was true even at the beginning of Beta when Zerg players went just Roaches. The strategy then was: I'm going to open Roaches. For my midgame I'm going to get Upgrades, Roach Speed, Burrow and Tunneling Claws. For my late game I'm going to get Roach HP Regen while unburrowed (since removed). Its not hard to see why people thought Roaches were overpowered at the time, but the key thing here is that even in this simple case he has ideas of when he wants to tech; its part of his core strategy.
This is not to say you won't have to adapt to what your opponent is doing. If your strategy is the TLO ZvT hellion harassment, maybe you build some Roaches before teching to Infestor. However, the key to execution is knowing that even before you've completely shut down that harassment you need to be upgrading your lair and getting Infestors. Just because you adapt doesn't mean you have to compromise your strategy.
I'm going to give you one last example: Terran MMM, because its actually well defined:
- I'm going to wall off and open 3 Rax
- Once I have a nice clump of units I'm going to expand
- My mid game is going to be getting Stim and Medivacs
- Once I have a sizable force I'm going to start breaking map control
- For my end game I'm going to add on factories and start pushing siege tanks.
The reason MMM is such a popular build is that is very easy to do and it has a very nice mid and late game transition. Once you have your 3 Barracks you think about Expo. Once you have Expo you go to Medivac for your Stim. Once you've moved to late game you're backing up with Tanks. At all points you are strong, and at no point do you stop using the unit producers you currently have.
So there are a lot of factors here and I'll try to break down them into categories:
- Why do I get Zerglings
- When are Zerglings good
- How do you use Zerglings
The short answer to your question is: Zerglings are not necessary (by any means), but they are a very useful unit in a large number of situations.
Why do I get Zerglings?
The question of why people get Zerglings is an important one so let's first talk about what Zerglings do well.
Zerglings are the most cost effective damage per minerals in the entire game. There is no unit which (for cost) is as good at tearing things down. Looking over my charts, Zerglings do a whopping 28.74dps for every 100 minerals (unupgraded) the next closest unit is stimmed marines at 20.9 (for reference a Zealot does 13, a charged VoidRay 6.25 vs armored and a Hydra 9.6). So when it comes to laying on damage there is no better choice. If we want to look at hit points, we see 35 for 25 minerals (a Roach is 145 for 100 minerals) so there is no doubt that this is a very cost effective unit.
Going beyond how cheap they are, Speedlings are incredible fast units (4.7 off creep, compared to upgraded Reapers 3.8 or Hellions 4.25) which give you a large amount of map control. This combined with their small size and high dps means they are very effective for harassment and hit and run tactics.
So when we look at why people get Zerglings it is for two major reasons:
- Cheap unit
- Great harassment
- Banelings
both of which the Zergling excels at (I know I put a third one there, but this is about Zerglings; Banelings are covered elsewhere).
When are Zerglings good?
Obviously they're a very effective unit early game, but this is largely a result of the problem: teching takes time. Instead let's look at when Zerglings aren't good.
Zerglings aren't good when your opponent is getting units which are effective against them. Reapers, Hellions, Zealots, etc, you probably know the drill by now. Because people often open with these units it definitely leads to perception that Zerglings are an underpowered unit. What's more any ranged unit that works in large groups (eg. Marine balls) really reduce Zerglings effectiveness.
Because Zerglings are a melee unit the "surface area" of your opponent's army is an important factor in how to use them. Shapes with a large surface area (eg. a line) allow for many Zerglings to attack at once. Shapes with a small surface area (eg. a circle) allow for fewer Zerglings to attack at once. As a result, even though you may have many more Zerglings than he has Zealots (for example) if he clusters his Zealots together they can over come these numbers (think the Spartans in the movie 300). Terrain also plays a role here. If his units span a gap, then you have even less surface area your Zerglings can access.
Finally, Zerglings really suck in small numbers. This is the biggest mis-usage of Zerglings I see. Someone builds 24 Zerglings and sends them to fight 10 Zealots. I know 24 Zerglings feels like a lot, but its really not. Think of supply for a second. 24 Zerglings are 12 supply, while 10 Zealots are 20 supply. While supply isn't the greatest indicator of a units effectiveness you can quickly see why you really don't have enough Zerglings for this fight. If I'm going Muta/Ling and I want half my force to be Mutalisks, half to be Zerglings, and to have about 60 drones, that means I should have about 140 Zerglings. If I have 40 or even 60, I'm just not building enough.
So Zerglings are good when
- You have a lot
- You can pick where you fight
- You're not against units which really hurt them
How do you use Zerglings?
There are really two parts to this question: the Micro and the Timing.
Zerglings require a decent amount of micro. Despite what you've seen with AI pathing and auto surround, the AI is actually pretty dumb. If you stick your Zerglings behind Hydralisks who are attacking, they'll get stuck back there. Additionally, Zerglings will target whatever is closest to them (frequently Zealots) instead of what they excel against. Finally, a Zergling will run right up to a unit and start hitting it. While this works well for units standing in place, any unit on the move means the Zergling will get in one hit before it falls behind. It also means that its brothers now have to run around it to hit the target.
Reference Material
Notice how the Zerglings just run up to the Reaper and only get a hit or two in before it gets away? Now imagine, if you will, if instead of attacking the Reaper the Zerg player had moved (not attackmove) behind the Reaper. As the Zerglings spread out in a line (as they do when moving) they would be in a line passing the Reaper. Now when the Zerg player attacked, the Zerglings would surround the Reaper, preventing escape, and do more damage, even if it did escape).
While this Micro seems relatively trivial it is essential to proper Zergling usage. It not hard to practice either. Get used to moving your Zerglings (not attackmove) past your target and as they pass by, then attack.
Timing is another tricky thing with Zerglings. Because they require so much surface area attacking a large MMM ball (or an entrenched position) is rarely a good idea. Instead, find opportune moments to run your Zerglings into his mineral line, or kill of proxy pylons, or attack his MMM ball when its spread out. You'll find your Zerglings are far more effective and do a lot more damage.
Take Home material go to 34m 19s
So should you learn and start using Zerglings? Without question. Having another unit in your arsenal is always a good idea. However, Zerglings have some intrinsic properties which make them extremely useful and your play will benefit from them.
Best Answer
Ok, so it sounds like you actually have two problems.
I'm going to address the second one first.
Don't expand if you're still dealing with harassment and large armies. Expanding is very much BW style play and largely is designed around the concept: Expand when I have Map Control. If you don't have Map Control and you try to expand, what's going to happen is that your opponent is going to shut you down with some small task force (like 12 Reapers or 8 Muta). You're going to respond with your whole army and he's going to pull away with little or no losses. So if you're losing expansions to harassment, chances are you just don't have the map control to expand.
Next you talk about how you're losing larger armies (40% to micro). Almost everything your army does requires 0 attention from you, except combat. If you're engaging in a situation where you will lose (split army) your response should be retreat to somewhere you can win. If this means sacrificing an expansion, sacrifice it. If you're walking into an unscouted position, retreat. Scouting is a far more valuable use of your time in these situations than micro is.
Now, if you're losing because your Zealots are in the back and your Stalkers aren't blinking, and you're running into storms, then the problem you're really having is how you macro. You should be building and organizing your units as part of your macro. If you're building your Zealots behind your Stalkers and leaving them there, that is bad macro.
Finally, if you are engaging a scouted opponent and thinking that you should win, then give over most of your attention to this battle. Perhaps build things via hot keys only, but don't move your screen for more than a second. Big battles require a lot of attention, and you should give it to them.
You also mentioned harassment in a couple of contexts so let me give you some good advice. Photon cannons are not designed to stop harassment, they're designed to delay it. It is far better that his Mutalisks spend the time to kill 2 photon cannons, than you build the 8 photon cannons to stop this raid. Use defensive structures as a delaying tactic.