My understanding is that marijuana isn't even addicting in the strictly chemical sense, the way even alcohol and tobacco actually make permanent changes in your brain that cause dependency. So it's really one of those things you can go through a phase in life experimenting with and then give up when you grow up. The main concern I have usually hear against it is that it is a "gateway drug" that will lead to trying scarier stuff, but it seems to me that making it illegal is counterproductive, then, because that puts it on the same slippery-slope for the experimenter as the harder drugs. It contributes to the counter-culture appeal for those at a teenage level of maturity going through a rebellion stage, while not appreciably cutting down on accessibility. The sharp decline in smoking over the last few decades obviously wasn't the result of throwing people in jail, but in smoking losing almost all of its coolness. (In America, anyway; Europe's way behind the curve there.)
Plus, one criminal conviction can ruin the career future of a promising college student, possibly turning them from productive contribution to less savory activities to make money.
With violent criminals getting frighteningly short sentences due to our overcrowded prisons, it seems to me to be a matter of priorities. Let states decide if that's really where they want to spend their prosecution and incarceration resources. I know if/when I become a prosecutor I don't want to spend my time dealing with people like my sister's lame friends.
The mild correlation between use of pot and use of "scarier stuff" can be entirely explained by a pre-existing inclination to alter one's mental state ... which accounts for the fact that there is similar data for alcohol and tobacco being "gateways."