MMR penalty for abandoning games is stupid because:

#1 - If you win despite abandoning, it means you played well enough to win and get MMR. If that is not enough to justify +25 MMR, then MMR should be reworked so that only the players who carries their shit team gets MMR.
#2 - If you lose, you will lose MMR anyway.
#3 - Low Priority is enough as a punishment. Decreasing MMR is just adding salt to the wound.

Not to mention:

Forcing players to make bad plays to gain a tiny bit of XP, costing them their teammates' win.
Encouraging bad behavior (feeding) instead of abandoning.