Guest characters in fighting games are nothing new, with the first one popping up all the way back in 1993 with Fighting History's Karnov. These days, if you don't have a guest character in your game then you can't sit at the table with the cool kids. It's gotten to the point where we even have entire games built around watching characters from dozens of different franchises punch each other in the face, like Super Smash Bros or J-Stars Victory VS.
If you really want to outdo your competitors, you gotta bring out the big guns. That's why we've been lucky enough to see characters like Freddy Krueger and Leatherface popping up in Mortal Kombat games over the past few years, because nobody loves guest characters more than Ed Boon himself.
In the face of fierce competition, many studios have had to really pull their finger out when it comes to interesting and, more importantly, surprising additions to their rosters.
Some studios, though... they may take things a little too far. No one looks at the character selection screen on Super Smash Brothers Brawl and thinks "You know what? I want to play as R.O.B. today", and though we shouldn't dissuade publishers for thinking outside the box, just maybe, there is such a thing as going too far...