Some styles are alleged to be bad because they evoke sinful attitudes or reactions. "Listening to that kind of music would just make a person angry/worldly/lustful/etc." That is, people have certain sinful attitudes or dispositions, and certain styles of music could only worsen those attitudes or dispositions.
i'd like to suggest that this charge has been common and resounding among various forms of cultural arrogance and elitism. What generation hasn't been berated by the older generation that new music is sinful or wanton? The last generation of the Romantic Era thought the Jazz of the 20's was profane and spiritually harmful. The World War I generation disapproved of the "oldies" rock of the 50's. People today in 2009 think many contemporary styles are evil and wicked or wanton. Notice too that once people become the 'older' generation, their music becomes the 'safe,' 'morally acceptable' music, even though it was precisely the music the previous generation criticized them for. [I'm telling you, this pattern doesn't stop. I work in a guitar studio, and the classic rock of the 70's and 80's is becoming the new 'safe' music that's perfectly acceptable to teach young children.]
Not only age-elitism, but historically, ethnic and socio-economic elitism has been behind this "evil-style" charge. When people of lower classes and lower education have certain favored styles of music, those of higher classes judge those styles to be base or profane or "low-class" (ever heard anyone say that before?). Do you think this kind of biggotry ended after the civil rights movement? Even now, some parts of poor, urban, black culture are judged to be sinful or base in the eyes of middle-class whites.
Notice here the same phenomenon as before--what once was base and evil over time becomes a standard by which to judge other cultures. For instance, notice how "spirituals" were very popular to use in worship over the 80's and 90's even though they originate from the slave classes of the 18th and 19th century.
So if the older generation had their style judged as evil by the previous generation, why does their style get to become the best or favored or the one by which all younger generations should be judged? If higher classes begin to accept as popular what was the styles of the poor or ethnic minorities, why were those styles ever looked down on in the first place? Answer: Cultural Elitism.
Couldn't that very well be what happens among church folk? One socio-classed and/or aged culture dominates, and so judges all others as unacceptable to greater or lesser degrees. Song styles are "good" or "bad" depending on the degree to which those songs conform to the preferred-styles of that dominant culture.
Remember, some people say things like "no one could possibly have their heart in the right place and like that kind of music." Oh yeah? That's likely the very same thing someone said a generation ago about the preferred styles of the person making that comment. And give it enough time, someone a generation from now will use that now-despised style as a 'safe' and 'acceptable' style by which to judge the musical preferences of the next generation.
Thus, these charges are subjective. There's no official, fixed platform on which a person can stand and judge cultural styles as good or bad. How, then, people in church legimately accuse each other of using or liking styles of music that are inherently bad?