If you have a feeder, there is a chance of winning.
If you have an AFK, there is no chance. (unless the skill level is severely outmatched, such as some high level guy playing low level, which generally doesn't happen due to the matchmaking algorithm except in a smurf situation or a situation where friends of different playing levels/ranks queue up together and the matchmaking algorithm gets confused.)
With a feeder, if the rest of the team is doing ok, and we tell the feeder to play cautious, farm jungle, etc, in the team fight, that little extra that the feeder brings (whether it's in damage, or absorbed spells, etc) can make the difference between a won team fight and a lost team fight. Even if the team isn't doing all that great, that little extra damage or meatshield is very valuable in the teamfight.
A lot of times people have rough early games, but turn out ok in the mid/late game. Or rough early/mid, and power spike late game.
(Actually, a common fallacy I see a lot of people make is that they have a rough early game and then they simply give up, which is silly because although the early game does matter, it doesn't dictate the outcome far as much as people think it does xP)