I think there is a similar core issue (not problem) in both mega-genres, that they tend towards extremity - so anything that represents a particular extremity (

I think there is a similar core issue (not problem) in both mega-genres, that they tend towards extremity - so anything that represents a particular extremity (e.g. slow despair - Doom, guttural aggression - Death, shrieking hatred - Black) is obviously going to be identifiable. But then since they are defined by extremity, anything less than the most extreme examples or with more competing influences than the purest examples has to be classified in a different way. Hence melodic death metal, DSBM, post-black metal, sludge, grindcore, etc... Extremity also breeds community, so you get definitions influenced by the kinds of people who listen to them. Would you want a ton of people expecting gabber turning up to a generic hardcore night? It's a very different feel. Similarly you wouldn't want people who were expecting Depressive Suicidal Black Metal (DSBM) turning up to a second wave black metal gig. Personally I like sub-genres, I think they're useful for people.