I'm going to piss off a lot of people by saying this, but metal fans in general tend to be really hung up on the whole identity thing and tend to sort of be musical snobs. You see, creating these sub genres makes it appear as if metal is far more complex than the series of power chords and pinch harmonics that it really is. It's all about image and what band or sub genre expresses the image the listener wants to portray. There are more posers in the metal genre fan base than all others combined. I have nothing against metal music at all, just making an observation as a long time musician. Bring on the flames metal heads...
The reason for these subgenres is not because they are begging for attention. It's really because there is a necessity for all of these subgenres. Black metal sounds completely different than post-metal for instance. It's necessary. Heavy Metal was a established as a genre a while back, and has gone through evolution. However, I would like to correct the genres that you have listed. Pirate metal doesn't actually exist. It's just a term used for bands who use pirate lyrical themes. Pagan metal is an umbrella term for bands who sing about paganism. Christian metal fits into the criterion listed previously as well. Mathcore is more so a subgenre of punk. Screamo is also a subgenre of punk. Now with musical diversity, it is necessary to categorize different musical types in different genres. It makes way for ease of access.
sub genres help when I'm trying to describe a band, or when I'm looking for more of a particular sound, but it's a double edged sword because I think it limits people's imagination, everything has to fit perfectly in a category, but I find the best bands often do not fit perfectly in a category. some genres are superfluous duplicates of other genres, for example Folk metal is a real genre, as you can imagine it's tradition folk music + metal, but pirate metal is just folk metal + lyrics about pirates, that's not really a genre when you think about it, and neither is Viking metal.
it was about the time that everyone wanted to be something special/different/unique
