I do not believe the fact that it has been illegal has ever been a factor in peoples' choices to smoke it or not. I do not believe legalization would create more users. Its not like everyone is waiting for legalization so they can all run out and buy it. The most effective way to reduce usage is to demonize it and make it socially unacceptable like they did with tobacco. Legalization of its purchase, possession, and usage, would not change the laws concerning driving impaired, and would not change the number of people who choose to illegally drive impaired so those arguments are a moot point. They may need to find more creative ways to test for it, because now they usually settle for the possession bust, and do not pursue driving dope impaired.
I do know that when I was a minor, it was much easier to buy dope than alcohol simply because those who sold alcohol legally wanted to be able to continue while those who sold dope were already breaking the law and so did not check ID's. I believe if it were sold by licensed retailers, the black market would dry up, and it would be less obtainable for minors.
I believe adults of legal age have the right to destroy themselves if they choose. It is arguable that all immorality, and irresponsibility affects others, but it does not mean we should entrust our own judgement to the government to ban with law. Sure it fries one's brain. A lot of former MX riders are in wheelchairs or worse. Do we really want to go down the road of using the law to protect us from ourselves.
Finally, legalization crushes the black market. Prohibition made the Kennedys wealthy because of black market pricing. While the drug cartels will still be peddling cocaine and smack, I would not feel bad about denying them all the proceeds of underground pot peddling market.
I've never been a pot smoker, and have no interest in becoming one as I think its destructive to one's own self, but I also support its legalization as I think the benefits outweigh feared negatives.