Starcraft – Why don’t Terrans ever seem to use medics

It sounds like you're building a large army and using it to gain map control, but then are falling prey to things like drops and backdooring.

If you have map control your first thought should never be: Crush my opponent, as you're essentially sacrificing an advantage (map control) for an unsure victory (because he doesn't control the map doesn't mean he can't take you). Instead your thought should be: take the map. I think you'll find this change in philosophy will lead to situations where you have a much more powerful economy than him and you're able to kill him with attrition.

Now if you're falling prey to things like BD here is a quick check list for you:

• Are you scouting? an Observer should be the first thing you build out of Robo. If you don't go Robo, a Hallucination works just as well. Sometimes scouting means sacrificing some Zealots in a pointless charge to just get inside his base.
• If you're scouting, are you seeing it coming? Overlords on the edge of your base, Starport with Reactor + 3 Rax, all of these are indicative of certain strategies you should watch out for.
• If you see it coming, how are you responding? If you have map control that means you should be able to respond quickly. Shutting down these back door attempts is often a bigger victory than a straight up conflict
• Finally, Do you have map control? If you don't have map control that means he does, and if he's beating you with map control then your problem is not "how do I deal with a base trade?" but rather, how do I regain the advantage.

Surrounding

The problem that you have with the kiting bio balls is that it moves away from your army, surrounding the army is a proper remedy and disallows him to kite further or run away. So, instead of the default way of attacking, you will want to get a good surround first. Move your units past his army, and once they are in a surrounding formation just perform an attack move.

This is demonstrated in this video, you will first see the Speedlings attacking them in a default way but then he properly surrounds it by walking past the army and attacks again which denies kiting.

The same counts for Banelings, as long as you don't kill your own Zerglings in the action. ;-)

Another thing that can help is spreading creep, because you can surround faster on creep...

A word of caution about Zerglings against a Bio Ball, which is the opposite for Banelings:

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.

— Tzenes' Answer on "Importance of Zerglings to the Zerg armies"

Kiting

Also known as stutter stepping, can be done in different ways, and is a personal choice.

In this thread, a video and map can be found to practice stutter stepping.

Your proposal looks fine enough. I usually switch between "attack move" and "move"...

Stationary unit demolitioning

Stacking Mutalisks and attacking when they are all in range is efficient, watch out for Thors though!