My answer to this, from another post:
Everything can bring bad ideas. I fail to see why its a good idea to bring characters exclusive to one port of the game and frustrate players who could want them, instead of bringing someone to be on all platforms and appease everyone. Besides, even from a tournament perspective this would be a bad idea, the characters would have to be banned since players wouldn't buy all platforms just to practice against them.
It has, actually. But nothing with extensive movesets (of course, that wouldn't be necessary considering that Spawn, the Star Wars characters and Ezio had lots of moves made for them), think about... pretty much anyone that uses a weapon in League of Legends, for example, I bet Riot Games wouldn't mind lending one of their characters, maybe they could even time that up with a crossover skin for those who bought the game.
I stand firmly at your side.
This is one of the main issues, actually. It would bring nothing but frustration and it could even potentially reduce the visibility of the guests, since they would end up banned in tournament play. Again, why not add a single character to appease everyone?
"Just a guest character" brings a lot of hype for every game, specially in some specific cases. You can bet, for example, that the TMNT are the most hyped DLC for Injustice 2. This should be done in a non-frustrating way.
Not only him, between Demon's Souls, the Dark Souls trilogy and Bloodborne, we can have quite a long list of potential characters. Its not like we're out of options.