It does not clean your lungs! thats one of the stupidist things i've heard in a while.
Your lungs have a mucus lining that protects your aviolies from all the crap/dust/smoke that you breath in, but if theres just too much crap in there then it becomes toxic and you have to get rid of it (the phlegm you cough up), when you cough this up it leaves the surface of your lungs exposed to carsonagenics and other such poisens.

And come on it's common sence that breathing in smoke is bad for you.

Ps. where on earth did you get that fact about the coughing up phlegm being a sign that your lungs are clean and healthy!?

And as for the incresed cell death, if a cell in your body is unhealthy it will be replaced, weed kills off healthy cells that will need more energy and more nutrience to be replaced.

I'm not saying it has not possitive heath benifits, i'm saying it has more negitive effects then possitive ones.