I like Waluigi because he's a total ****head. I used to not care about him so much, but he's grown on me. Plus I have some friends who would be quite happy to see him included. Total ****heads, mostly. Funny how that works.
If he only showed up once or twice it'd be a different story but he's been in more games than basically every other candidate people mention. And yeah, they're spin-offs, but so what? Not like Smash is canon to anything, and we already have all the main Mario cast other than Toad.
I do understand. Waluigi support used to be almost entirely ironic. But it hasn't been that way for a long time. Now the ironic to sincere ratios have basically inverted. And he's an inessential character to his series. But Smash is well past the point of what's "essential".
I can see the parallels in that Sonic has a ton of ****ty friends who largely appear in spin-offs as well, many of whom I wouldn't really want in Smash. But in a Sega Smash game nearing triple digits, sure, put em in. They're notable Sonic characters after all.
One of the things not to underestimate about Waluigi demand is it extends so much further than the normal bounds of our little internet hovels, wherein average Smashers go "they should add Waluigi". He makes much bigger waves within the casual component than most of the alternatives we raise.
^see? Positivity. It is possible. I'm basically a rainbow on ketamine.