I'd first like to point out that marijuana has no permanent negative side-effects at all, and the worst temporary side effect is that very heavy users experience a temporary IQ drop of about 5 points, which fades if they stop using it. I'd also like to point out at this time that 5 points is very little. I've never done any kind of drugs or anything, and I took 3 IQ tests in the space of 3 years and the results varied by more than 10 points.
Pot should not be legal simply because tobacco and alcohol are, but because there is no reason for it to be illegal, and so much money is wasted in policing marijuana use (70% of drug arrests are for marijuana).
Alcohol and tobacco should be illegal because they cause damage to your body, and in the case of alcohol, can cause you to become violent. Marijuana on its own cannot do this, as a matter of fact, from the users I've known, they tend to be nicer people and easier to get along with if they are high.
For the good of society, marijuana should be legalized in place of tobacco and alcohol. So many people are of this belief nowadays, that I am certain it will be within the next 10-20 years.