To me (and this is an issue, since many people will never feel this way), a character that lacks even or bad matchups is detrimental to the game.
As an example, I'll show scenarios involving character choice, and why MK limits the game (in my opinion)
In this scenario, MK loses to no one, or goes even with anyone. Essentially, there is ZERO, NONE, NO reason to not pick meta knight. You might as well play chess, where everyone is limited to the same options (MKs options). More MK players are coming up, and I believe it will continue until this ame is almost entirely MK dittos. While there is nothing wrong with a ditto of any kind specifically, when the entire game is based upon this, you remove an aspect of gameplay from Brawl: the character choice.
Compare this to a (hypothetical) metagame I am about to illustrate:
Snake is the best character in the MK banned metagame. Picking snake is the most logical option, but is not a complete trump. D3, Falco, and Pikachu all have favorable (though not by much, it is there) matchups against Snake. So if I expect a tournament to be loaded with Snkaes, I can pick one of these characters to gain an advantage. And because of this, people can expect to Play Ice Climbers to have an advantage against these characters, because I expect people to prey on Snakes weaknesses. But in turn, I lose to Snakes, since snake beats ICs. There are of course a bunch of other viable characters, but this goes to show a point, it adds (or rather MK removes) an aspect of gameplay to Brawl. Instead of having an entirely surefire way of picking characters, it adds the aspect of characters choice, and character choice is a skill in itself.