"Yes, one death band has keyboards and plays more harmonies than the others. They sound more melodic. It's not really enough to spawn a genre.."
It's a lot more than one band, that's why it's a genre and not just a one-time thing 
I for one find the many different genres incredibly useful when you want to discuss bands with other people, so you can at least give them a very good idea of what the band sounds like.
Oh, and as for the Iron Maiden, Black Sabbath and Judas Priest thing mentioned earlier, you're right. They are all Heavy Metal, anyone claiming otherwise is silly. But (and here you'll probably be annoyed) each of them added another genre to it, respectively NWOBHM, Doom and Speed.
I'll never know if it was worth the pain, but I still loved it more than anything in the world - it was my life.