Yes, it's worth it to pick another jungler. It does not necessarily have to be a counter-pick though, which probably needs some explanation:
- The advantages you gave for having a jungler hold for your team as well.
- What you labelled as 'more effective' does not really require a jungler at all (coordinate your team to kill the enemy jungler [wards!] or steal a buff)
- Your downsides are too situation dependent:
- If behind enemy lines, you better know what you're doing. You have to be sure that your champ can live up to the enemy jungler and you have to make double sure that all other enemies are on their lanes. While it is a downside, it's one for which the risk can be dealt with.
- If their jungler is higher level than you, then either you have to improve your jungling, or you got killed/hindered before. Shouldn't happen for the first creep round, unless someone messed up their early game (see below)
- same as 1. Do not get killed in the enemy jungle. The enemy jungler shouldn't be able to kill you (or you shouldn't enter his lair!) and if you get ganked it's your fault for losing track of your enemies. If you suspect only a single enemy missing that might come for you, you better be on your way back to your jungle.
Anyways, double jungle teams are quite a common thing, because neither team wants to ignore the advantages gained by having a jungler. However, it also means that your team should know how to deal with that situation. Here's a few tips for general two-junglers-play:
- Prepare for more ganks on both sides. For both junglers it's usually safer to gank, than to visit the other side's forest.
- Make your jungle starting save. Not getting harassed while you get your first buff is extremely important. The team should realize this and protect you. Place your mates into the forest access points near the river to watch out for early ganks. Only let them leave, once the buff is save.
- Of course, you can turn this around too and try to gank the enemy jungler with your team. Not just a single counter-picked champ, but 2+ champs.
As you can see, two junglers are possible, do make sense, and generally lead to very fast and intensive gameplay. Beware on random groups though, as a two-jungler game demands a lot more from all players and failure is easy to achieve in that setting.
First, let's clear up a misconception.
Armor Penetration gives you a constant %damage bonus, regardless of how much armor the enemy has see here.
It doesn't matter if your enemy has 300 armor or 15 -- penetrating 10 of that will have the same effect on total DPS increase.
That said, let's take a look at why Armor Penetration is preferred, even with its linearity.
Attack Speed is easy to acquire via items.
Armor Penetration is not.
You can more than match the effects of attack speed runes with a single dagger - 420 gold.
The cheapest item with Armor Penetration is the Brutalizer at 1337 total gold. By this metric alone, Armor penetration runes are worth approximately three times that of attack speed runes.
It is easier to use items to stack Attack Speed over Armor Penetration -- but to get the highest DPS, you want to stack both.
Given that the amount of Armor Penetration available via items is strictly limited, if one's goal is to achieve maximal values for both Armor Penetration and Attack Speed, runes for Armor Penetration are much better than that of Attack Speed.
Best Answer
I am going to make some assumptions that, both teams are equally high level skilled and make few mistakes.
A high elo team is not going to be stacked up where they are at a disadvantage. Thus reducing some of the AOE abilities.
Non AOE Team Perspective
What if there is a position that makes it favorable to the AOE team? You back up and find a different position.
How can I shutdown the AOE team? Having a tank and off tank may be your answer. They will put pressure on the glass cannons while your DPS is spread out. This will reduce the AOE damage as well.
You may be able to counter an AOE team with, AOE stuns(ammumu, veiger), silences(soraka), disables(lulu,blitz), tanks.
Engage with an AOE stun on the carries and focus them down if at all possible. Use your silences to stop skills from being cast. Use your disables to make the enemy less effective. Your single target DPS should be able to burst down carries when they are stunned.
AOE Team Perspective
If you are an AOE player, such as Vladimir, you probably do not want to ult on one person because that would make it less effective in team fights.
You want them to be stacked up on baron, in bushes, etc.
You do not want to use your ults and AOE abilities on tanks because they aren't your worst threat.
Conclusion
Counter pick to deal with abilities in the late game, but you'll still need a viable team for early game as well.