he was in Mario Kart 7 as a playable character
and there's a giant R.O.B. in F-Zero GX
plus R.O.B. was a homage to Nintendo history, being a peripheral that launched with the NES.
I think because of that history he more than had right to be a playable character in Brawl.
plus judging from that trailer for Wii Fit Trainer the Balance Board will have some appearance in the game, and that's just a peripheral too.