Ah, I see this discussion has been already ongoing! How vain of me not to think someone else would have raised this issue! :)
So, to step in and offer my two pounds fifty's worth, I can see a need for SOME sub-genres, reading the reponses here. I love SOME metal, but not all. F'r instance I'm a loyal acoylte of Maiden, I like Kamelot (I know you said they weren't metal Jack, but bear with me), Adagio, Sabbath, some Motorhead, Lizzy and so on, but then there are a LOT of bands I would not personally listen to. I may have mentioned somewhere else that I can't hack what are apparently described as "death vocals", (but which to me is just growling), so bands like Opeth, Haggard, Slipknot etc will never grace my Zen. Doesn't mean they're crap, just that I don't like them and have no desire to hear them.
So if I can differentiate MY (as it were) metal from THAT metal, by use of a sub-genre (let's say, Classic Metal --- oh no, it would have to be written Klassik Metal, wouldn't it?) :), then that's not too bad. But reading the list of sub-genres here, and I have to agree with boo-hoo, some of those names are AWFUL ("Drowning in a sea of mourn"? That's not even a sentence --- a sea of mournfulness, maybe.. Sorry I'll shut up before I piss off that band's presumed legion of fans), it does seem overkill.
I mean, can there be THAT many different types of metal? Someone else here said that metal seems to be one of the two only genres that seem to need this, and I wonder why? Is it an elitist thing? Oh, I'm into Viking Doom Proto-Foeutus HyperScare megaspeed Metal --- what are YOU into?
Or is there some other, deeper reason?
The other, linked question I guess is, what MAKES a band metal? Of ANY sub-genre? Is simply playing too loud a qualification? Looking ugly? Professing to worship the devil? Lyrics about death and speed? But not every metal band does this. So how do you categorise a band as heavy metal, BEFORE you even get into sub-sub-subdividing it into one of the exponentially increasing number of sub-genres?

Most sub-genres of metal are needed. Especially for things like recommendations. Black Metal, Death Metal, Doom Metal, Thrash Metal, Hair Metal, and NWOBHM or simply "Heavy" Metal, all have distinct sounds. Within some of those subgenres, there can be even more genre classification.

I agree with jackhammer, I am really disheartened when I call a particular band something, i.e. 'metal', and then someone turns around and says something like "they're not metal, they're <insert sub-genre here>." Why do we have to be so particular about assigning certain genres to certain bands and not moving beyond these tags but being very closed-minded about who is what? It's kind-of stupid. Different people will see music in different ways, and may see different music in different categories to others. It ****s me, too. There should be more focus on the actual MUSIC being played rather than the meaningless word being assigned to it.

the latest sub genre that tickled me was: Melodic Death Metal. Surely an oxymoron?

Ummmm....no its not. Death metal can have melody.

Then it defeats it's purpose. I know it DOES exist out there but it defeats it's purpose. Death metal was a more aggressive aspect of Thrash with emphasis on lyrical content, concentrated riffing, guttarall vocals and a general heavier ethos than Thrash could offer. Putting 'melodic' back into the equation is an excuse to soften the sound or create another pointless sub genre.

Having so many subgenres, it just seems like people are trying to differentiate similar music, rather than using labels to lump them together. It's pointless and just creates a narrow minded view of music if you like some but avoid the others when to the uninitiated they all sound the same.

So, by this logic, all melodic death metal is pointless and soft. I'm sorry, but melodic death metal is not pointless. Of course, there are many ****ty melodeath bands that seem to be more acquainted with power metal than death metal, but bands like Unanimated, Crown of Throrns, AT THE GATES all were melodic death metal. I think almost any metalhead would disagree that those three bands are soft and pointless.

'Soften' not pointless and soft. Don't worry by the time you are my age there will be another 50 sub genres and then you can berate all the kiddies about how many there are ;)

Why are metal heads so concerned with whether or not their music is 'soft?'