Well, you can't deny that the genres are useful, at least for conveying a general idea of what the band might sound like. If I describe a band as a "progressive black metal band", it would give more of an idea to the other person than if I just said "metal band". This could make it more (or less) appealing for the listener to decide whether or not to check out the band. Whether or not you consider this an advantage is up to your judgment...
I know I have been guilty of prejudice against bands based on genre before, notably prog, which I later began to enjoy - and now love. I still shy away from power, thrash, and all things glam. Maybe that's stupid of me, but I've yet to hear almost anything with those labels rightfully applied which I actually enjoy.
On a side note, has anyone really heard anything innovative in the black metal camp lately? You might say Ihsahn and Enslaved, but they're more prog these days.
</braindump>