Letting people leave early with no penalty makes it harder to get daily wins—if a player quits very early, the win doesn't count for XP, but it does count for MMR, with the result that if you play ones and then encounter multiple quitters in a row (which happens way more often than I'd like) then you're at higher MMR and now have to beat tougher opponents in order to get your wins.
Or, more likely, you lose a few times before getting back to having a reasonable shot at an even matchup, meaning that the quitters have cost you a bunch of time.
Also, it's just really annoying to encounter people who quit early, and this might slow smurfs slightly as well (since they have to spend longer on the way down).
No idea if they made this change on purpose, but I hope they keep it.