The amount of tar in smoking straight pot is far worse than tobacco. The problem is, I doubt you will find many people that smoke pot who do not smoke tobacco.
Also the THC remains in your fat cells and afflicts your brain function (Although it has not been determined exactly how) for several weeks even after you stop smoking it. I know when I smoked often, even when I was sober I would feel to be in somewhat of a dreamstate. I could function well, such as with driving and such, but my thinking was somewhat off.
I'll try to find some documentation on it later, but there was one very fraudulent study done that showed marijuana causing death of brain cells. They tested the effects of jest ten (or 80, or 8 million) marijuana cigarettes a day on laboratory mice. The reason I say it doesn't matter if it is 10 or 80,000,000 is because 10 joints to a mouse would be like a giant grocery bag full to a human. No one would ever consume that much. The mice had brain damage, too, but it was from carbon monoxide poisoning, same as they would get of forced to inhale the smoke from burning cauliflower or even tobacco. With rules of science like this, I could prove Iceberg Lettuce is deadly - if 300 lbs of it were instantaneously air-cannoned into the victim, er, ah, subject in question. I guess I've rambled on long enough,
Dakmar