Smash 3DS is in a weird spot. It doesn't have the vast single player content of the previous game, it doesn't have a roster as massive as the next game, and there's another version of the same game on Wii U. It was incredibly cool to have a completely portable Smash game, but even that's something the next game does. So you'd think, well why even bother, what's there to go back to?
I actually greatly prefer this version to its Wii U counterpart though. The classic mode here is my favorite in the series, choosing different routes based on difficulty is a nice shake up and maybe my favorite classic in the series? Smash Run is so much fun, I still come back to it all the time and wonder why Wii U and Ultimate didn't bother with it. Same could be said for some of the stages here that didn't return in Ultimate, Rainbow Road and Pac-Maze. For some reason, this is also the only Smash game to save the last skin you used so you don't have to swap through them to get to your favorite.
Sure it loses some of the impact of being a portable smash these days, but for a handful of years this was the coolest thing ever. And at the end of the day, Smash is Smash, and it's so much fun, even if it's not the best in the series at anything in particular.