Not true. Marijuana has about 1000 more cancer-causing chemicals than your average cigarette. Keep in mind, tobacco is a natural plant too, it's the process of preparing and smoking the tobacco that it becomes dangerous.
Also, for those of you who say that marijuana doesn't cause you to lose reaction and motor skills when you're driving, you're wrong too. When I was in high school, a friend of mine and his mother were killed by a driver high on marijuana.
Marijuana is a gateway drug. Now, it may not be a gateway drug for everyone, but in my experience I have SEEN it be used as a gateway to get people introduced to illegal drugs. Legalizing marijuana is not going to solve the nation's drug problem, either. You're still going to have cocaine, heroin, meth, PCP, LSD, and other drugs that people are still going to pimp out their children for a hit. Most people who have started on the "harder" drugs started with marijuana and/or alcohol. Plain and simple.
Prohibition didn't work because 99% of the country drank some type of alcohol. Less than .01% of the country REGULARLY smokes weed. Also, notice how the organized crime syndicates didn't go away after Prohibition was repealed. They simply grew and became more dangerous in other trades later on, especially with the unions.