KILLING PEOPLE IS NOT AN EXPLOIT, HOLY CRAP.
You're either not very smart, or are intentionally misunderstanding what I said.
The problem occurs when your team overall is doing very good, but there's one shitty player that keeps feeding the enemy. Things like that are largely outside your control. Even if you hang out with them a lot, and babysit them while they auto-attack and try to protect them when the other team comes to prey on them, you won't be able to help things like them still having 2500hp at level 10 and dying to a single burst.
Bad players often times catch so much flak that they just quit and are replaced by AI, which tends to be even more stupid. Furthermore, AI joining the game usually results in more quits, which completely ruins everything. Last night I played four games total and I think I spent more time waiting in the game lobby than in the game. And that was because in each game, people kept leaving as a result of hopelessness that is caused by one bad player dying over and over, or AI players.
This may be a team game, but it is really unfair when you are punished for things that are outside your control. Since you cannot pick your teammates in pantheon and skirmish games, and cannot figure out who is good/bad in custom games, often times it's a gamble. I don't want the game ruined simply because I was unfortunate enough to be sided with a sucker. Same goes for the other team. I don't want it to be an easy win simply because the other team had a retard.
If the game had clan support then you could say "well, you already knew who you were playing with, and that they were bad, so deal with it" but that's not the case. The vast majority of people do not roll with premades.