Well, I have to say that those people may have had those problems regardless of whether they smoked pot or not.
But, pot does NOT clear your lungs. There is 7 TIMES as much tar in pot as in regular ciggarettes.
As far as cancer goes, one should concider the fact that both Bob Marley and Jimmy Hendrix both died of mouth and throat cancer, and no coincidence they were both well known for their pot smoking. Just something to concider.
However, I had a friend who used to have severe anger problems, and smoking pot was the only way he was able to remain calm enough to be a normal part of society.
I had another friend who had a genetic seizure disorder, and he was able to stop taking his (very expensive) medication by smoking pot. Hasn't had any seizures for the last 10 years now, about.
However, there is some element of psychological addiction. An ex b/f of mine would talk about nothing but pot almost everyday. It was to a point of obsession. That's just not normal.
The point is, it has good points and bad points, but i think you should only smoke as much as you need to accomplish what you're trying to do (and not make being stoned all the time one of your goals!)