Most of the people I've heard bitching about having to play as the Arbiter are partial to 'realistic' shooters and don't really like science fiction much to begin with, I've noticed. They're the ones that say they can 'relate' to Master Chief because he's 'teh uber-Marine of teh FUTURE!!!!11!'
I was talking about Halo 2 with a coworker at lunch here, and he started bitching about "how the game went all sci-fi".
Me: "uh, what, the game is set 500 years in the future, of course it's gonna be 'all sci-fi'"
Him: "what"
Me: "you've been fighting ALIENS with LASERS the whole game, why is it so 'sci-fi' to suddenly play as one"
Him: "i dunno, it just seems gay and unrealistic"
Me: *BOGGLE*
The dude loves Tom Clancy games, though, and thinks Splinter Cell is the greatest achievement of all mankind. He also listens to 80's butt metal in an endless loop all day, so maybe I'm giving him too much credit.