Mii fighters are practically useless unless you can use them online, I don't understand why Nintendo doesn't just cave in an allow them, you already can get the most hostile people though abusing names as chat in Smash, and lets not forget Mario Kart 8's Voice Chat... then there's the "blind spots" of miiverse...
The only place I can't see mii fighter applicable are at tournaments, and for glory matches, keeping them out of for fun is an oddball for me, how hard would it be for them to add a checkmark filter for "allow mii fighters".
Then again you can use mii fighters online if you play with friends, so you can technically use Mii Fighter over wifi, just not publicly, its probably just another case of "angry nutcase parents" and Nintendo being scared of being called over a mii with a butt face, then again why would a child that can not handle seeing a poorly made butt face of a mii even be playing smash online?
Also I haven't bought a single piece of DLC yet, the characters are about the price of the game itself and that steers me away from it.