I can see both sides of this argument being valid. The prospect of entirely banning a character is a big one.
My argument for banning goes something like this:
Suppose you have a complicated game of rock paper scissors where, instead of paper beating rock 100% of the time, this occurred with probability 3/4. So every time you play a match with someone else. You are expected to win this game 50% of the time, not taking skill into account.
Suppose you add in Spock. Spock has a 50% chance to win against himself, and a 60% chance to win against everybody else. it is clear that when I play Spock against a field of rock paper and scissors I'll win 60% of the time.
Does this mean, however, that Spock will win all the time? No, but does it mean that Spock is more likely to win than any other strategy in the game? Most certainly.
If metaknight were to be banned, it would take on a more rock paper scissorsesque form, and I would predict that many different characters would start to arise at the tops of tournaments.
Does this mean that metaknight is "broken?" not really, he just has been shown to have an above 50% chance against every other character.