"But Guy, i know people who listen to a certain kind of music; and it clearly DOES seem to have negative effects on them. So surely that style of music is no good for anyone, and especially not good for use in a worship service, right?"
Let me say upfront that my posts do seem to confound two issues: styles of music one listens to for entertainment value, and styles of music that the church should or shouldn't use in a worship service. i'm trying very hard to stick with the latter, but i know i'm touching on things that concern both.
So first, Jesus taught us that nothing which goes into a man defiles him, rather it's what comes out that defiles (Mark 7:14-23). It's not as though there's a perfectly good, unspoiled person who rather haphazardly chose to listen to a CD, and as soon as he pushed 'play' all these black, ghostly mists flew out of the speakers and into his ears and possessed and corrupted and diseased all his moral faculties. A person likely already has sinful attitudes and motives in him. All music can do is affect what's already there. But music doesn't miraculously create sinfulness in a listener.
Second, remember i'm speaking strictly about musical style. This is not about lyrical content. It's obvious that certain lyrical content could be unequivocally unsuitable for entertainment or worship. But this discussion is all about style--just rhythm, tone, melody, and harmony. If you're going to say that it's bad, you've got to show how those mere elements in that arrangement and combination are bad or spiritually harmful.
Third, of course some musical styles affect some people negatively. And if that were the case in some worship-music-selection situation, and you knew it, then you would have an important and weighty argument why that church shouldn't use that style. However, when someone proposes songs of a new style, you very likely don't know whether that style arouses something unspiritual in them. For all you know, given their particular cultural and sub-cultural memberships, it could very well be that their preferred-style of music does for them precisely what your preferred-style of worship music does for you. It could be that such style does enhance all the right attitudes and dispositions for them (reverence, joy, humility, gratitude, etc.).
So one or a handful of cases where a sinful person likes music that scares you does not serve as a basis for saying that people wanting different styles of music in worship must be headed down a dangerous path or be ill-intentioned or anything of the sort. Is it possible? Of course, it's possible that someone's preference for a certain style of music in worship is a bad thing (maybe for them it's a selfish choice only meant to satisfy their tastes and nothing more). But notice, it's just as possible that someone's preference to keep a style which they find traditional could be just as selfishly (and therefore, sinfully) motivated. Thus, again, it is not the style of music that is inherently suspect or dangerous.