It wouldn't work, for all the reasons as above plus more:
1. It is, when you get right down to it, a Superhero film, which is the most hit-and-miss genre in the history of Film.
2. It is, when you get right down to it, a Superhero team film. Of all the Superhero team films ever made, there has only been 2 Great ones (X-men 2 and The Incredibles), 2 good ones (X-men 1, the very first Teenage Mutant Ninja Turtles), and the rest have either been average-at-best (X3, for example) or so bad that they must never be allowed to be discussed ever again (Fantastic Four series in general).
3. It would be a hogtied mess visually, just look at, say, the SSE videos where Pokemon trainer and Lucas are standing next to each other. They are both human, sure, but they look totally different. While this is nifty in a game, it would stink in a movie.
4. Every other reason given.
But, what the heck, here's the voice cast for SSBB (* means voiced character in games or cartoon/anime):
Charles Martinet as Mario*, Luigi* and Wario*
Samuel L. Jackson as Captain Falcon (the coolest character in smash can only be voiced by the coolest in Hollywood)
Ikue Otani as Pikachu*
Makiko Omato as Kirby*
Orlando Bloom as Link (think more "Will Turner" then "Legolas from LOTR")
Charlize Theron as Samus Aran (I think she may have actually been mentioned once or twice as a candidate in the perpetually-in-development-hell Metroid Movie)
Shia Lebouf as Ness (would also make a good Ness in a live-action Earthbound, which, of course, would never, ever, ever, ever, ever, ever, ever, EVER be made)
Kazumi Totaka as Yoshi*
Takashi Nagasko as Donkey Kong*
Jim Walker as Fox Mccloud*
David Hayter as Snake*
Nicole Mills as Princess Peach*
Mike Madeoy as Falco*
Ryan Drummond as Sonic the Hedgehog* (although it would end up being J. Griffith because of the stupid 4Kids controversy)
Random Beep and Boops as Mr. Game and Watch
Veronica Taylor as Pokemon Trainer* (would probably be replaced by whoever is doing Ash's voice now)
Rachel Lillis as Jigglypuff*
Antonio Banderas as Metaknight (One of the few good things about the Kirby anime was Zorro-like voice for Metaknight. But only the idea was good, not the execution, since there is only one Zorro-like voice that can be taken seriously: Zorro)
Stephen Colbert as King Dedede (The Anime made the mistake of parodying Texans. Dedede is not a Texan. He is merely a holier-than-thou, easily fooled, easy-to-anger idiot. Sounds like Stephen Colbert's portrayal of Stephen Colbert to me)
Maybe I'll fill out the rest if I ever feel like it.