^
Playing highly punishable characters will cause you to learn more about mindgames, reading your enemy's reactions, and throwing out laggy attacks in anticipation of their moves. I don't consider myself to be particularly skilled with MK, but even I can see how spacing is a bigger part of his metagame than, say, Ike's.
One difference that other people may be able to relate to is the idea of reaching plateau's. With ike, the plateau is extremely easy to reach. Once you understand his move set and how to use them, there's not a lot of technical room to improve. Instead, the game becomes about reading reactions which is extremely hard at lower levels of play. You'll get punished and again, no matter how tight your technical skills become until you can read your enemy well.
Metaknight, on the other hand, is relatively hard to punish. Because of that, your technical skills will naturally evolve more. Spacing yourself to avoid punishment is far more practical with metaknight than with someone like Ike. Because of that, you'll naturally start using attacks in a way they won't be punished. You learn that tornado spam is highly punishable against a sharp enemy, etc. Because of that, I feel metaknight is the best character to refine the technical side of brawl. Without a keen understanding of your enemy's attack patterns, one can still do reasonably well with metaknight. You're not going to be winning tournies without it, but you'll be able to place from time to time. With someone like Ike, it's simply not possible against reasonably skilled players.
Also, will everyone please stop arguing that knowledge of metaknight still puts him above low tiers? Of course that's the case, he's top tier for a reason. The subject at hand relates to tournament viable characters. A ganon player can know everything there to know about MK, and if his skill level is close to a MK, they will still lose the match 9 times out of 10. Ganon simply isn't viable, as is about half the cast at a tournament level.