I'm a fanboy concerning the use of "Discuss." to end forum posts. That's because you should never do it.
Anyway, something like League vs. DOTA 2 or PvE vs. PvP sides of any given MMO seem like more contentious debates than MK vs. SF.
Ultimately I think it's stupid but it also extends to iOS vs. Android, Canon vs. Nikon, debates among sports fans, and so on. It might be somewhat more prevalent among the gaming fanbase because of our frequent internet use and overall age/maturity range, but not so much that I think it's a problem that can really be singled out with any effectiveness.