I would really like to hear this answered by someone (Ideally multiple people) that wants to keep MK in the game. Just an answer to whether MK being added to the game improves it in any positive way or not, if it had started without him.
1) MK provides another character for people to play. We don't know how this would affect tournament results/dominance in the metagame mainly because we don't know how a non-MK metagame would play out over the course of two years. Obviously he makes some characters more viable and some less viable. However, how he actually plays out in certain region's results can't be said for sure. As we see now, some regions have had major problems with MK (NM, Gtan, etc) while others have done just fine with him and/or a ban of him at this point would reduce a lot of attendance numbers assuming all of the MKs quit (eastern MW, Atlantic North, etc).
2) MK forces some characters to adapt new playstyles to cope with him. From what I can tell, tough match-ups help a character's metagame grow quite a lot since they have to try to make them more winnable. Take for example, Diddy vs. Marth. In 08 and early 09 this was a match-up largely assumed to be 60-40 Marth or possibly one of his worst match-ups. In I think April of 2009, Le_Thien brought in and elaborated on the element of Diddy's mid-range game, and argued that because of it the match-up was even. While this caused some controversy, a couple months after, after many top Diddys beat top Marths and Diddys in general got better with the match-up, it was considered to be even.
Usually match-ups fluctuate or they become less in a character's favor over time. Match-ups that were at a point considered to be 65-35 one character may be 55-45 or 60-40 now. Granted, match-up numbers are subjective and mean differently from person to person, but in general match-ups have been more doable. Usually the only cases of the opposite happening is when 0-somethings or infinites have been discovered (for example, ZSS infinite on ROB). And after coping with these tough match-ups, some characters are able to apply that playstyle to many other characters (mid-range zoning helped Diddy with match-ups like Luigi and MK as well).
Having a top character like MK that provides a tough match-up for nearly every character might make the other character's metagames grow.
However, all of this is—what I said and what Crow! asked—is a very lightweight argument because it's all theory. We don't know what would happen in a two year non-MK brawl game, so we can't answer what adding MK would improve/subtract from the competitive community by adding him in.
This also just seems like picking at strings. We can theorycraft all we want, but this isn't analyzing what's actually happening now in the Brawl community. A question like this shouldn't have an effect on MKs ban because what MK does to the competitive community right now doesn't focus on "what-if" questions.
Also, salabo, since you asked that many people answer that question, I'll ask that you and many other people ask this question. As I've said multiple times, I just want to see where people stand on this.
What problems are MK causing to the Brawl competitive scene right now?
What is going to be accomplished by banning MK?
Namely it's the first question that I want people to answer, but the second kind of ties in with it.