I'm not a tournament player, so I'm not going to advocate a specific position, but I think there's something worth considering here.
Basically, I think it's fairly uncontroversial to say that no one would intentionally make a character really unbalanced. Now, of course, this doesn't really help the discussion too much if looked at in one way; when you're designing a game it's nearly impossible to fully predict what strategies are going to work competitively.
Looked at another way, though, it does open a certain possibility. Almost every time there was some kind of video while the game was in development, it was 4 players with items, not one on one battles. I think we can reasonably infer that the developers expected(rightly) that 90% of their target audience would be playing this game most of the time in 4 player mode with items. As such, when ensuring the game is balanced, they're going to be looking at this mode.
I only have my own, casual experience to go on here...but then again there are few item tournaments and no free for all tournaments, so that's all anyone has in this context...but anyways, I've noticed that light characters in general die much more quickly when 4playItem mode.
For the short of attention span, bottom line of this rambling mess is this: It's entirely possible that Meta Knight, or even other characters, might be grossly unbalanced if the game was designed with most of the focus on making sure that play seemed balanced in 4 play.
Please, if this is way off the mark, that's fine, just don't start a flame war. I'm just putting the idea out there, and we are talking about a VIDEO GAME, so no need to get tempers high.