Funny that you say that....
When you don't get the context of which it's being discussed.
Which, for your information, was what SmashChu said:
Waluigi is interesting. He used to be disliked during Brawl. Now, he is defiantly a contender in terms of popularity (you can see the various poll for that). I think people have warmed up to him, which is why there is demand for him to be in a main Mario title. That's the only thing holding him back.
Specifically the last part.
He's saying that the only thing holding Waluigi back is lack of a Super Mario (I'm using that to refer to the platforming series now. Just makes more sense). game appearance.
I'm merely giving other examples in how it doesn't take a Super Mario appearance to "release the chains" so to speak on what is holding him back for fan request to pull ahead. It could be a Luigi's Mansion appearance. It could be a Wario Land appearance. It could be a WarioWare appearance. It does not need to be a Super Mario appearance (and one would make no sense anyway).
I do not find him unworthy because of being a spin-off character, nor do I feel that he needs one of those to have a fighting chance.
I feel, however, that one of those can bring his requests to be higher than the current "tie" between him, Toad, and Paper Mario. (Although, a Wario Land/WarioWare appearance would heavily lower his competition, since he'd be competing for a Wario spot instead of a Mario one...)
Aside from that, if Pokemon can get an unworthy Pokemon like Jigglypuff, and Mario can get a spin-off character like Dr. Mario, then I fail to see the problem with Waluigi other than the typical "he's just a skinny Wario" BS.
EDIT: Oh, he's not important? Then tell me how Jigglypuff is important to the Pokemon series and why it/she should stay. And no, the "Original 12" argument doesn't count.
He's spin-off only? Didn't stop Dr. Mario, who was even planned to be re-added for Brawl.