While some games that shine best as multiplayer experiences still have enormous and unique single player campaigns, such as Halo, the Super Smash Bros series is unique in that it is built from the ground up as a multiplayer / party game. Single player modes usually serve as structured training regiments for multiplayer battles. Sure, Melee had the "Adventure Mode" and Brawl had "The Subspace Emissary," but did anyone buy those games for those modes?
"Multiplayer" is not quite as inclusive as it might sound. This isn't Jenga after all; some of the best multiplayer experiences require that you lock horns in a battle of strategy and skill, not random chance. This can turn some people off, but it can also create an incredibly engrossing fight. This holds true in the Super Smash Bros series, where button-mashing can only get you so far. Though a competent understanding of the moves and attacks of a character is required, the power-ups and level dangers help keep the hot dogs on an even keel. If you can round up three friends who are familiar with the title, then you will undoubtedly experience gaming bliss.