I think that's a matter of raid composition and balance. Simply put, there's more DPS than tanks, so you can safely lose a DPS or maybe even two, assuming there isn't a really tight enrage timer. It's sort of stuck like that, too, because it'd be quite hard to script a 10-man encounter where you need 3 tanks, 3 healers, and 4 DPS, much less an entire dungeon around that kind of ratio.
Odin'sLaw, you might consider checking the mirrors at this page:
http://www.wowwiki.com/Patch_mirrors I tend to use that when the downloader is being its usual unreliable self.