Well I can still see why smoking weed the conventional way (bong hit, doobie) is bad for you, its the same as tobacco in that sense, you are burning the substance and the resin/tar is going to your lungs...
but if you use a vaporizer, I think this negates most of the bad effects, and can actually have a lot of health benefits for a wide range of people. It strictly 'vaporizes' the THC and leaves the plant substance in tact. Plus I know tobacco has thousands of chemicals added to it, while weed doesn't (as far as I know at least) :P