Funny, my brother and dad used to play Mario Superstar Baseball sometimes, and my dad was interested in Waluigi because he'd never seen him before. He mentioned that he was at a gathering (forget whether it was a party or a golf retreat or what) and one of the other fathers played a Mario sports game with his son, and they soon were talking about Waluigi. This actually inspired a username I used at another website, "Meister Waluigi". I would've used Reading if I'd planned on staying there longer, but as it turned out, I'm still there, and had my username changed a long time ago. :P Looks like Waluigi was a bit of a phenomenon for a while.
I don't mind Waluigi; I mean, whether they do it or not, he always has the potential for character development. Also, wasn't he in one of the microgames in Wario Ware: Smooth Moves? I haven't played that game, but I remember hearing it somewhere...