Music styles are subjective to each persons tastes, and their personal walk. Its between God and that person and its no one elses business. If you feel something is wrong, avoid it. But it doesn't make it wrong for everyone. The bible makes it clear there are areas in life that are up to each person, and two people, with differing opinions, are right, in their own walk.
Personally i've been listening to non-mainstream Christian music since i was saved, when i was 14. I was even listening to Stryper before that, haha. I currently have over 1500 albums of Christian music, mostly stuff that is not mainstream. Metal, punk, hardcore, techno, goth, reggae, rap, industrial, whatever floats my goat in a rain coat.