Let's put it this way; What black metal DOESN'T cover some form of philosophy?
Astoundingly little. Even the 'oooh, it's so fucking COOOOLD' school embodied by Immortal is laced with spiritual and philosophical overtones: indeed, most black and death metal can be argued to be Nihilistic. Hell, read anus.com. As for other genres, About 90% of decent MDM (ie mainly the old stuff) is philosophical in nature: the first two In Flames albums are mainly about Astrology, and most of their middle period albums are about the failings of society, once you chip through Anders Friedens lyrics. Dark Tranquillity and At The Gates as well. Edge of Sanity aren't exactly MDM, but they certainly fit the bill: songs ranging from everything from arguments against religion (The Sinner and the Sadness, Enigma) to pro-gun control rants (Enter Chaos) spatter their albums. Huge portions of viking and folk metal espouse the philosophies of natural religion and criticise our modern society and it's conventions: hell, every single band Martin Walkyiers ever touched has ended up spewing reams of dense philosophical and social commentary, inventing a few genres along the way. Just because it's often more of a subtext doesn't mean it's not there.
Yeah, and don't get me started on doom, mofos. Remember that Black Sabbath were all about banning the bomb, smoking dope and even a touch of religion a couple of times. Maiden never shied away from a good political track either, though they personify the main difference: when Bob Dylan wanted to comment on the hypocrisy of the military and the slide towards Nuclear war, he wrote 'Masters of War'. When Maiden wanted to comment on the same thing, they wrote '2 Minutes to Midnight'. All more veiled, and with more solos.
Erm, ramble ramble.
Emo, eh?