Genres are only a necessity for conversation. When you are explaining a new band to a friend, the first thing they're going to ask you is, "What kind of metal is it?". This is so they can get a rough estimate of whether they will like it or not. If I told a friend about Zyklon and how they are death/black/thrash, and he is a huge power metal fan, chances are he's not gonna like it. If I simply said, "Zyklon kick ass!!!", he might have a totally different expectation than what it actually is. That's why we have genres and sub-genres. IMO, to say "They aren't black metal anymore", i.e., Emperor, that doesn't automatically mean it's utter shite. Who cares what genre it's in; if you like it, listen to it. If you don't, don't.