Probably the biggest reason to get SSBB will be to avoid the wrath of all the SSBB fanboys on this forum who will tear anyone a new one who doesn't own "The Holiest of Holy Game Of All Games In Existence Or That Will Ever Exist EVAR!!!111oneoneone", otherwise treating them as a lower life-form who doesn't even deserve to own a Wii.
I suppose I understand some of the appeal here, since 99% of the users on Wiichat are young males who grew up with the likes of Street Fighter, Virtua Figher, Mortal Combat, and other fighting games. I played them a bit myself (although games like Zelda appealed to me more) so I'll probably pick it up for the occasional play, since it seems like it'll be a well-polished game. But it won't spend as much time in my Wii as other games, and I'll probably be the only one of my friends who bother playing it. Most of them prefer the racing games, Mario games, Harry Potter, Pirates of the Caribbean, etc.