7. Left 4 Dead - Everyone
Valve CorporationIn the event that a zombie apocalypse were to ever happen, you'd want to be in a safe place, be able to defend yourself and, most importantly, be surrounded by other people who had your back. If you've ever had the pleasure of playing Left 4 Dead, you'll know that if you play it with friends or other people, you have a very large chance of survival as you gun down or hack apart thousands of the undead. If however, you decide to play Valve's excellent survival game on your lonesome (with only AI controlled teammates to help you), you'll quickly find that survival isn't very likely, because your "friends" will be just as brain-dead as your enemies. Zombies are supposed to be dumb, (that's how humans always find a way of surviving), but when your fellow computer-controlled survivors show about as much urgency as a hungover college student, you'll wonder why the option to have AI teammates even exists. The problem wouldn't be so bad if you only had normal zombies to deal with, but with special infected also present to bolster the ranks of the shambling flesh-eaters, your grim fate will already be sealed. One particular example that always comes to mind is a particular type of enemy - the Hunter. This thing will single out a target and then jump onto them as if they're trying to lick jam off of their face. When this happens, the player afflicted with Hunter-on-face syndrome is unable to move or help themselves - another must help by killing the offending monster. No problem if you're playing with other humans, an impossible task if your friends are AI controlled numptys who will get themselves killed long before they realise you're in mortal danger.