Lots of people don't like crossovers. I like them.
The biggest problem with crossovers is that they make main characters into secondary (or at least marginalized) characters. If you have the super-cool main character dudes from two different IP interacting, the real question is: Who Will Win? Who is cooler? Whichever is chosen, fans of the other IP will feel gypped.
There are ways around this. For example, if one of the characters is a bad guy, you know he'll lose. It's just a matter of how cool his showing is before he goes down. You can have characters who excel at different areas, or make characters not interact.
But that's in movies and other noninteractive media. Games don't have to worry about that crap.
Games were MADE for crossovers.
Each character has a specific game dynamic. Each character can be balanced and added to the game as a whole. You can let the player choose a main character, and next time through they can choose a different character.
If you're a big fan of SnobKillerX, and Lucy likes Victor Telepath, you'll naturally pick those characters for your first run. And you'll think to yourself, "Wow! Cool! This is exactly as it should be!"
Games are the ULTIMATE platform for crossovers, so long as your balancing is good.
Why do I bring this up? Evidently, there's a Namco & Capcom crossover game coming out (but not for us, of course).
Now, I don't actually LIKE either Namco or Capcom games. Street fighter is among the most boring games I care to play, and Darkstalkers is only marginally more interesting. But Capcom is the KING of crossovers, and so I really like their crossover games. I like getting to play as these stupid characters against other stupid characters. WHY? WHY AM I INSANE LIKE THIS?
Yeah, it's really stupid when Wolverine takes on Ryu. Who cares whether Lilith or DigDug is stronger?
I looked back at my posts on action, and the answer came to me.
All of these characters are ESTABLISHED THREATS. Even characters like Bomberman and DigDug are long-standing icons who are known - at least to somebody - as being unstoppable and strong. They have histories and they have power.
Compare this to a game with new characters. Those characters only have the history you MAKE for them within the game. Girl X defeating Guy Y really doesn't do much, because we don't really know what their power levels are, and they have no pre-established patterns.
Furthermore, there's an element of excitement to crossovers. The characters have proven power, yes - but the scale and effectiveness of their power is unknown. It's a little like a gamble, at least for a while. How will Storm fare against Little Red's firepower? Lilith against Akuma? Watch two known patterns clash - what will happen?
THAT is why I like crossovers. The new take on old and powerful patterns. :)