Thanks for clearing that up, Strell. I didn't know the source, but I knew it was a rumour.
I personally think they should definitely have as many characters on the cover as possible. Not everyone knows Super Smash Bros - every Nintendo fan may know it, but I'd be willing to bet dollars to doughnuts that way more people recognize Pokemon, Mario, or even the Legend of Zelda than Super Smash Bros. And the group that's going to be the most ignorant is also going to be the most impressed by Snake and other third party characters, so I think Nintendo should put as many characters on there as possible. Every gamer should spot at least one character they love on the cover when they walk past.
Having said that, I miss the days of simplistic boxart when you could get away with nothing more than a logo or maybe a logo and an illustration (see Super Mario Bros. 3). I'd like to see more simplistic box art in general, but I think Super Smash Bros. is one title where more is definitely better.