Categorizing by genre can be risky and dangerous because almost nobody gets the genre of a song correctly unless they have a very fluent background in it. I myself listen to a good dozen or two genres of music that the ignorant masses would simply define as "heavy metal", despite the apparent variety in styles. I myself am not even innocent of this, either. I don't know the correct genre for a good portion of the groups I listen to.
On the other hand, generalizations of genres would certainly act as a filter against completely incorrect genres. If I wanted to search for a song by Wintersun, and it were categorized simply as metal, stuff like Sheryl Crow, Nobuo Uetmatsu, U2, Blink 182 etc would most certainly not appear in the search, so I could see it working.