Miis don't serve any problem. In fact they are the perfect form of customization. People complain about Mii names and looks, but you already have the ability to change your name and looks are a minor nitpick considering you still look like a Mii.
Miis are characters just like any other Nintendo character. They have their own characteristics and abilities. Unlike during Brawl they have so much potential from games like StreetPass, Wii Party U, Wii Play, PilotWings Resort, Wii Sports, Wii Sports Resort, Wii Party, Wii Fit U, and the consoles themselves could be implemented. The fact that Miis have their own move set and charateristics shoots down the whole issue of "people online always playing as Miis" because unlike Smash, you didn't have to worry about Miis being different from the rest of the roster.
The fact that Smash is on Wii U & 3DS makes it a good idea to add Miis. I'd also love to be able to earn things to add to your Mii's physics like wings to have multiple jumps and armor to make you heavier and increase your offense.