I wouldn't call them useless: you use them to describe music to someone. Or to recommend something similar.
On the other hand I don't feel the need to separate one subgenre into one million other subgenres since it can get confusing and annoying, but there are people that like to have a very clear image of what they listen to.
With your logic, Korn and your favorite band are the same metal lolz
Who says you should like every band in your favourite band's genre?
This is about as dumb as it gets. It's pretty obvious why genres exsist.
Example: Why do they call it pizza? everything should just be called food.
Sure, but when taken too far it's like hearing people say things like:
"Waaah, thiz pizza has tomato sauce, not pizza sauce! You can't call that pizza!"
Or "A pizza should be fattening and unhealthy and every slice should have a pump of lard just like Pizza Hut. If you make one with healthy and organic ingredients, it's not trve pizza."
Grank
"Something putrid in smell or essence. Puts the witness in a state of disbelief as to why something this horrible would exist."