In fact, I think it should be legalized and treated the same as alcohol — and cigarettes. We could then get some good research done as to the actual benefits.
It all comes down to the dose, and since most marijuana users probably don't light up a doobie more than once or twice a week it's unlikely that they would have an elevated risk of lung cancer. It would likely show up in the heavy users, though — people who smoke the stuff daily. At that point they are starting to get to tobacco use levels.