Listening to the CDs of Within Temptation (a lot) I've been thinking -- musically it makes a lot more sense to have female singers in metal. You have all this infernal grumbling going on in the guitars/bass/drums section and you really want a soprano voice to cut through this, to be distinctively audible on top of all the grumbling.
Traditionally there have been two solutions:
1) the extended guitar solo
2) male singers who can't sing very well, but who can emit high-pitched screams (the name Ozzy Osbourne springs to mind for some reason)
I am intrigued that so few bands have hit on the obvious solution of hiring a female singer. Well, in many cases it may have to do with the fact that a band owes its existence to the inflated ego of the frontman = singer. But still. I think this is a field where an end to the gender discrimination would improve the quality of the product. A lot.