The reason people don't do it is because it's practically impossible to do with out sounding shit.
I fucking love metal
obviously dubstep too.
But it usually sounds cheap and gimmicky to try to combine them.
Distance manages to bring metal into his sound sometimes without it sounding gash but that's more down to the fact that he just likes metal I guess not that he's trying that hard. If you go out of your way to combine the two it will always come across as kind of stupid.
Don't know if you checked the King Cannibal album on Ninjatune last year, but I think that might qualify for some heavy doom influence. Not so much metal guitars, but definitely in the spirit, even down to the cover.