There are lots of threads about this issue, but about this issue i have a diferent not such straight view.
I understand the problem, and why ppl complaint about leavers, but only to a certain extent.
Consider the following examples:
1) a player(s) looking for games to raise his score, enters a game and on the other team someone leave after being killed the first time. That players team get weaker with a AI playing for him, no doubt. That player should get his score afected, no doubt about that also. This is the comon case ppl complaint about and makes sense.
2) some friends, imagine 4 guys, want to do tests about strategies with diferent demigods, builds and items. They will for sure create and leave several games without finishing a single one. Should any of these players be penalized? I dont think so.
3) a "casual" players finishes his dinner around 22h and wants to have some fun, play a bit, finds a game 4x4 and enters. No problem there, maybe the game will end in about 1h:30m that will make 23h30m when it finishes time to go to bed. But imagine that his team is losing, considerably but being able to pospone defeat for an extra hour, the dude needs to go to bed to get up soon in the morning to go to work and the only thing he wants, since he sees no chance of winning, is to forfeit the game (+1 loss) and go to bed. In his mind the game is finished and there is no point in delaying the inevitable. Why should he be marked as leaver, he admits defeat and forfeits... (chess permits forfeit for instance)
I'm just saying that not everyone is a ragequiter. Some times there is no point in playing a game till the end, its a waste of time or a e-peen demo for the winning team.
If some of you played WC3 a LOT, WC3 TFT and NOT DotA, after game number 2000 you know when you are going to lose and forfeiting at that point was a "decent" thing to do. Specially if you were in a Blizzard tournament. Some times you would play against a turtler, someone that knows he is going to lose but delays it as much as he can and you would lose precious time, time that you needed to complete more games to be elegible to the next tournament phase. And that dude just ruined your streak because he didnt leave "when he lost", no time for more games, not enough games to continue IN tournament.
No one likes ragequiters, specially in a mixed team of people that dont know each other and the quiter just nailed a big LOSE on your team. But there should be ways to quit/ forfeit/ admit defeat/ smth.