Smash wouldn't feel the same to me without it's single player content. I got more interested in other Nintendo games than ever by reading up on trophies of so many different characters and items that weren't playable or usable in previous games.
Give me Adventure/Story Mode along with the usual Classic and All Star modes, and we're golden. I agree that multiplayer aspects are the most important part to the game, but the franchise is a celebration of Nintendo games, and nowadays, iconic video games in general. Single player modes brought me into the feel of it all, and things like trophies add something special to it, I wouldn't ever want them missing from a Smash game, even if I don't read their descriptions as often as I used to.
Smash is a multiplayer game first and foremost but I wouldn't want to see it lacking in single player content.