Doing otherwise is really just a gimmick of sorts. It doesn't add anything to the game except a cameo.
So? It's a cameo, but as long as they have a well-designed skillset what does it matter?
Well in the grand scheme of things, it really doesn't. I say that to preempt any argument you may make to the effect of "It's just a game." Yeah, it is just a game. Doesn't mean we can't have opinions on it. You may not make that argument, so that's really more of a safety net.
But for a video game that's supposed to showcase a specific company's franchise characters, why would you want to import a character from a different company? Give that skillset to one of your own characters. I'm willing to bet Sony has at least 30 characters they could use for a game like this before even needing to bring out the obscure ones, let alone needing to import the star franchise character from another company into their game.
Soul Calibur has a history of doing this.
Soul Calibur wasn't named "Namco Fighting Super Awesome Gametime," either. It made no claims to be company-specific. The name "Playstation All-Stars Battle Royale" kind of implies that the characters in the game will, you know, come from the Playstation platforms. Not Nintendo or Microsoft, but Sony.