When it comes to crossover fighting games (but not necessarily just fighting games), how do you people feel about guests? The PS3 and Vita versions of Street Fighter X Tekken will feature Cole McGrath from the Infamous series as a playable character, and he is neither from Capcom or Namco.

Crossovers of this kind can already be controversial, but are guest characters better accepted in them than they are in an individual series? Or must crossovers be "pure" as well?

I just want to add that guests in crossover games do not nullify said games' titles and goals. Street Fighter X Tekken will remain a Street Fighter/Tekken game, as most of the characters are either from Capcom or Namco, very specific series from these two at that. A few characters from unrelated series aren't enough to justify changing the title to accomodate them. Even if you had two or three characters from the same unrelated series, that's not exactly much compared to say, eleven MK characters and eleven DC characters, using MK vs DCU as an example.