The only problem I have with R.O.B. is that it isn't nostalgic at all. Old, yes, but not an enduring memory upon many a gamer. But no one really knows what nostalgia is supposed to be, and they all go by "Anything for the NES."
To me, a perfect example of nostalgia would be Yoshi's Final Smash. It reminds me of Super Mario World and how much I played that game when I was little. Mr. Game & Watch is somewhat nostalgic as many of his moves remind me of Game & Watch Gallery 2, one of my old favorites. But I guess Brawl has a little of nostalgia for everyone--even people who loved Gyromite.
But for that small group, the cameo would be enough...
Edit: I guess it DID help out with the recovery from the Video Game Crash. Oh well.