This is completely opinionated, but the way I feel is. Marijuana is taught at a young age to be something horrible, something that will ruin your life. When in reality, it's not near to as bad as people portray it. Kids, or teens figure: "Hey, they told me this would ruin my life, and I'm doing fine, maybe the other drugs that they said were worse." Then thereon go to harder drugs, which I do see as a threat.
If they just legalized marijuana, said it was bad, but it is their choice as it will not kill them, perhaps more kids would actually heed the warnings of these harder drugs. It is easy enough to access now, if they made it legal it may even diminish marijuana use as they would tax it and make it cost a lot. Also, it would lower crime (No, I don't mean just lower crime because making less laws always lowers crime because there are less crimes.. Trust me, that made perfect sense >.<) I believe through the same effect that would make people try harder drugs because it was not that bad, people would also try bigger crimes. Also, many of the murders that occur every day are over drugs.
Edit: Upon reading the thread, I also agree with what Tradd said, there should be restrictions just like the ones alcohol have, perhaps an age, can't drive, none in public. There is a time and place for everything, people will still be uneasy about it after it's a law and not want to see it done in public. Would kids take up the smoking of tobacco if upon growing up they didn't see hundreds of people smoking it?
Note: I'm not saying I'm perfectly unbiased, but I don't even smoke marijuana.