Yeah I always notice this in England, the reason most people give is to conserve the herb as it's so expensive, but I think it's more just what they're used to, it's how they started smoking so they're habitualized to it. Also, no doubt many are hooked on the nicotine. I know a lot of people who used to smoke cigarettes, then quit and just smoke herb, but always smoke it with tobacco, either in a spliff or wrap up in fronto.
Personally i got used to smokin the backwoods blunts in Brooklyn, but I can't stand the spliffs with mixed tobacco/ganja. Better to not smoke any form of tobacco though. Unfortunately I am hooked on de fightin' cigarettes as well, so I can't play holier-than-thou on those who mix their herbs.