As someone who plays a lot of Dota style games,leavers should absolutely be punished. In any kind of competitive match without drop in/drop out style matchmaking, having people leave is a huge kick in the ass of the team they were on. It can basically turn the game into a cascading failure they have little chance of coming back from due to no fault of their own. People who ruin games for any reason deserve a loss, and doing so repeatedly is grounds for punishment, no matter the reason. If you have things randomly come up all the time, you need to plan better. If your internet is bad, then you need to strongly consider playing, as dropping constantly still ruins the game for others even if it's not malicious. Everyone should be allowed to have a good multiplayer experience, and frequent leavers are ruining that.