This could be useful on either side, I think. It seems more obviously useful for Magneto's side though. If the brotherhood is up to something and the X-men try to stop us, I'd turn them into cowards so they'd run away or just give up.
It's not a versatile power, since it's specifically "courage". Physical or moral courage, according to the wiki page, really only applies to story-central kind of things. Like, I can only really alter someone's fear to face an enemy/challenge. I can't just make them afraid of random things. So I'd only be useful during confrontations, which sucks because I'd much rather stay the fuck away and fart around with my superpowers on my own, but I'd probably have to come along to every fight just to assist the other guys.
My mutant name would be Courage the Cowardly Dog.