Tar in itself isn't necessarily "bad" for you. I smoke in the ballpark of 20 joints a day and the doctor tells me every year that I'm one healthy mother fucker. I even took it one step further last appointment and blew into the CFM tester. He said I have the lungs of a non smoker, but I've been smoking bud every single day for 9 years straight. I could be a special case, but I highly doubt it.
As for the "danger" of weed, non existent. In real terms, the damage weed does to your brain is so minimal that its almost impossible to track. They have compared weed smoke to thinks like sniffing a highlighter, drinking a full coffee (heart rate), downing a beer, and many normal activities. If you look at all the things around us (Wifi, UV, Radiation), smoking weed probably won't effect your life expectancy. Of course some people are more prone to have problems, but for the most part there really isn't any comparison you can make between cigarettes and Marijuana. One kills people and the other doesn't..lol.