The issue is trying to use genres as specifically as possible instead of being a broad category that covers a wide range of music.
Metal is the worst for this. The vast, vast majority of people would call everything from Black Sabbath to Metallica to Pantera to Death to Behemoth "metal", but the genre snobs need to differentiate it all for some reason.
Yet, we're perfectly okay to call everything from Blue Oyster Cult to Fleetwood Mac to Nickleback "rock".
I agree with your general point about metal subgenres but they do help me a bit. For example, I've got no disrespect towards the subgenres of nu-metal or the trve cvlt type of black metal but it's just not for me, so if I see something tagged as that I can easily avoid it. As I say, that is in no way a diss of those genres, they're just not to my taste.
I love metal, but maaaaan fans take subgenres too seriously. I once saw a bar fight in my very peaceful city where one guy was shouting "That's not sludge metal! That's stoner metal! You don't even know what sludge metal is!"
And I find myself doing that shit, too. I was talking to a buddy a little bit ago and realized that I had just said "I'm currently listening to a German symphonic folk metal band, but the vocals are pure black metal."
Thats extreeme metal elitism, but you also prooved the usefulness of genres. How else, without using genres, would you describe that german band as efficiently as you did so someone like me can understand it and say:
Specific genres are awesome to find new bands, that you like, because they sound similar. If I like power metal and you recommend me Behemoth, because I like metal, your suggestion has no value for me, while I probably would like another power metal band like Blind Guardian.