This would happen on a daily basis with democratic voting:
-everyone votes to ban someone just because they don't like them, even if they haven't done anything deserving of ban (like IronRoman spamming replays [ab]using unit X)
-making multiple accounts to vote
-people work together to gang up on someone (WmD clan votes to ban kingduce for example) MVGame
It would just be a popularity contest, and mods would spend too many hours to filter through the BS. IF it was used it would need to be like dialing 911- you better be calling for a legitimate reason or you could be punished yourself.