The issue of banning, as SamuraiPanda already pointed out, is pretty simple: if you're going to ban a character, the only situation that would warrant it is if the said character is so broken, that no other character has a fair chance.
Meaning that, when it comes down to tourney championships, every player from Ken to Chillen to whoever else is pro nowadays, is going to inevitably choose MK. All the time. And since that hasn't happened yet--nor does it seem likely to happen--then MK isn't ban-worthy.
MK is not as broken as Akuma from SF. You can actually APPROACH MK without getting ***** and locked down. MK, as far as I can see, just has overwhelming priority, edgeguarding options, recovery, and combos (in a game that has seldom). He's not broken. He's blatantly overpowered.
And Marth was blatantly overpowered in Melee. As was Sheik. And Fox. And Falco. And Ice Climbers. Yet (assuming you guys played Melee competitively) we still found a way around it--and we should do the same thing with MK. Banning is too extreme a measure to employ, when smarter playing/tactics will (mostly) suffice.