Refusing precisely due to the reasons you stated. People didn't learn to distinguish italics text from plain text and thus Urza won matches he shouldn't have based on GameFAQS Urza factor. Not dealing with that, unlike I would for, say, Jaya Ballard who at least has a tightly knit set of spells that isn't Urzatron.
There are stupid people (and smart people saying stupid things) in every match though. You're basically ignoring a whole field of evidence for a character that's already vague because some people can't tell the difference between obvious game mechanics and flavor text. Might as well ban any character from a video game because some people will inevitably argue game mechanics.
I don't even recall anyone seriously trying to argue for Game!Urza, though I do recall some people bringing it up to make a point. Where that point was, "this isn't even peak urza. you don't want to see peak urza. nobody wants to see peak urza." And I guess some people did it to try to make Urza seem weaker, via a whole "dies to 20 hits from a ****ing squirrel!" type argument. But whatever, it was never a big problem, in any case.
Gerrard Capashen, the soldier he genetically engineered, decapitated him while depowered and Urza never reformed to full even though he could have, instead donating his powerstones to Karn the silver golem. Basically he can regen except it has caveats.
Yeah, most characters with regeneration tend to have the caveat that they can only regenerate while they still have the ability to regenerate. >_> Yawgmoth removed Urza's planeswalker powers. He never got them back. And even if he had, he knew at that point that the Legacy Weapon was the only thing with a chance against Yawgmoth; that situation is not likely to come up within the confines of MPFC.
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"The courtroom is the garden of holy judgment. Those with lechery in their hearts should leave this sanctuary at once!" ~Franziska von Karma