New guy here, been lurking for ages but just couldn't resist piping up on this one...
First off, the usual genre disclaimers apply here. Despite the opinions of those who dismiss the genre, the variety of styles within means what works for one may not work in another. Iron Maiden is not Pantera is not Meshuggah is not Sleep is not Baroness. Shred-fest bands may be able to get by with only down-tuned guitars because they're (generally) not worried about groove.
One great thing about music is the infinite variety of ways to skin that cat, even within a genre as seemingly constrictive as metal. I've seen bands with no drummer. I've also seen bands with five drummers (Rotting Hills). The great Al Cisneros plays in Om, and they've done some truly heavy stuff with no guitars (see also Death from Above 1979). Lots of great bands have no vocals. If every band needed drums, bass, guitars and vocals, the world would be a far less interesting place.
But one thing a bass does, and that a down-tuned guitar can't (no matter how many strings), is 'thump'. It's that element of a bass as a part-percussive, part-melodic instrument to bridge the rhythm to the riff (to steal a phrase from the above-mentioned Al Cisneros) that simply needs long, thick strings. You can groove without it, I guess, but it's a whole lot more work.