A licence even affects reviewers. It's blinding them by virtue of the miracle of a good licenced game. I'm thinking that the score should have one point detracted if you're not a reviewer who's stunned by a good licenced game, and another point detracted if you're not a Batman fan. In my case, that leaves a 7/10, which may be a bit too harsh - the demo gave me a strong 7.5 vibe.
Why shouldn't a good, well used license enhance enjoyment of a good game? Would the X-Wing/Tie Fighter games be as much fun if you weren't zooming around in the beloved Star Wars universe? Probably not. That's what's potentially good about a licensed game, replacing a potentially chinsky original IP with something far more developed, inhabited by characters we've always wanted to be.
Based on design, mechanics and execution (from my experience with the demo) Arkam Asylum would be a really great game if you were playing Beefcake Darkshadow: State Hospital for the Criminally Insane. But it's that much better cause you're fucking Batman and you're fighting the Joker voiced by the actors from the great cartoon and written by the show's creator. We don't have to subtract points because these elements actually do make it more fun to play.
No question, though, if they had made the same game with a weak, generic character and setting your score deductions might be accurate. If by some chance they created an original IP as good as Batman, they probably wouldn't. But they didn't make those games, they made this one, so it's not really worth discussing.