I find that to be a bit of a hassle. It's much easier to say 'I dislike country music, but there is a possibility there is a song or artist I MAY like' than saying 'I don't like _____, _____, _____, ______, ______, ______....' and so on.
So yes, while all of music is a genre in and of itself, that still leaves the subgenres to compare and contrast different styles of music. I could say that Metallica and IOSYS are comparable because they fall under the genre of music, but since they have absolutely no similarities otherwise (IOSYS is a Japanese doujinshi technopop group that mainly does remixes of Touhou music).They sound nothing alike. That's why genres exist-- to make things easier, which goes back to my point: it's not prejudice to dislike a style/genre of music, you just dislike it and that's all there is to it.