The thing I think is important about a story mode is that it launches newcomers into the game and teaches them how to play. It's easy to imagine a situation in which a newcomer buys the game, can't quite get the hang of it during multi, and stops playing. Now, for most games, that wouldn't have any financial repercussions for Nintendo. However, from what I can see, Smash Bros number 1 promotional machine was the addicted gamers themselves, pulling friends in, getting them hooked, who in turn go out and get the game and possibly a system. In fact, it's how I got into it, and it partially sold me on buying a GameCube. So you don't include a story mode? Well, you may loose potential upstarts who could be the heart of your marketing machine.
Now, if they have another fun way of launching newcomers into the game, that's fine... but it's a very important aspect.