I agree with Null...
But maybe that's because co-op games are my favorite types of games... even crappy ones
It all started with Double Dragon for me.
I can see adding Multi-player hurting the score of Metroid Prime for visuals, and if it doesn't turn out good it could possibly hurt the score. But would it hurt the game itself? Doubtful.
I dunno, but a Co-op Metroid doesn't sound bad at all.