Well, I personally have no reason to play multiplayer. I don't plan on getting good in Smash so the whole competetive aspect is lost on me.
And my Online experiences have been pretty laggy. And the friends I have are not into couch gaming, I could maybe get them to play Mario Kart at best.
I do miss the Smash Run from the 3DS game. I would have a lot more hours if that was in the game :D
So I am slowly running out of things to do in Smash.
Do you play multiplayer against AI? There's no goal to work toward but there is limitless fun to be had with all the settings you can tweak and stages to choose from. I always enjoy inventing modes, though I wont lie it's more fun with real people of course.