Heya,
I'll be honest while I've noticed some players using warwicks I can't really say they've been in the majority. I've been to lots of metal gigs this year along with Download Fest and there seems to be a pretty even spread of Fenders, MMs, Ibanezes, Warwicks etc with the odd boutique bass thrown in, as I think would be expected in any genre really. I would venture to suggest some of the more influential bass players in metal use warwicks and since they get more coverage Metal is more readily associated with them. Who knows?