well basically I can agree with Zombi on most of this... you just can't generalize all Christians or on the other hand, all kinds of metal.
I'm both a very dedicated Christian... specifically Mormon... and a metalhead.
my religion comes first... sooo I use my beliefs to judge what metal I listen to.
but anyway... metal is an extremely diverse kind of music, sooo why not include Christian metal?
yea, there are some Christian churches that condemn metal and rock and whatever, but not all. which is in itself generalizing, because there's plenty of good metal even outside Christian metal. but some churches fail to see that... as you've said... they'll just tell off anything that they think is 'bad' or 'evil', when it's just not that black and white. which is why even I have a problem with that kind of thinking... but anyway, not the point.
it's not like the churches are changing their mind or anything... some still generalize. it's the people that know better that are forming these bands... that know metal and Christianity aren't complete opposites and can be combined. the existence Christian black metal isn't hypocrisy, it's more of a sign that both Christians and metalheads are starting to see outside their little holes.
and as for why to combine them... well, because we love both our religion and metal. they're both a huge part of my life, sooo of course we're going to figure out a way to make it work sooo we don't have to give up either. it may require sacrifice on the metal end... at least for me... but it's a lot better than saying 'screw it' and giving it up completely. I don't have to choose one or the other... I get both. and it's amazing. :]