There is anecdotal evidence on both sides of the fence. There are some new studies (source was in Denmark) that show medical consequences but not significantly more or less than cigarettes, with a couple of exceptions where it is much less risky than smoking tobacco, so you get down to how does it effect society. Mostly the criminalization of it has been harmful, not the actual drug. Criminalizing it has brought up a subculture that exposes people to harder drugs, and during the 70's it was often unintentional on the end users part, PCP etc. Pot has been called the great un-motivator for good reason also. I know a lot of successful people do use it, but a lot more that use it don't live up to their potential and end up contributing less to society than they could, to their own detriment largely. Society always needs "ditch diggers". I've lost a few friends that while in high school and even at the start of college had enviable grades in various disciplines and now work what are basically subsistence level jobs, the proverbial ditch diggers. I also know a couple of people that are highly successful and hold down high level jobs that use but they are the minority. What really surprises me is the number of people that don't outgrow it and become adults. When I went through my rebel without a clue stage I had long hair, ate mushrooms and dropped acid, but eventually I had to grow up and join society and honestly miss it not one bit. A little different but basically the same juvenile motivation.
Medicinally I don't see how it is more effective than the chemically pure versions that have been tested, other than of course those are therapeutically effective levels that don't get you stoned. There is a mental dependency that some people experience on the other hand, a self medication type situation. Unlike another drug used this way, alcohol, at least it doesn't cause you to drive through town risking everyone's life and limb. Still driving under the influence of pot should be treated criminally, impaired judgment and cars don't mix but it seems to be a lesser danger than driving drunk. A road side test would be needed, but I'm sure that is easy enough to come up with.
Personally, for rather personal reasons, I don't associate with people that use it, at least to my knowledge that is. Decriminalization would free up a fair amount of prison space, drug enforcement resources and generate tax monies from usage taxes like cigarettes do. Just as it is illegal to produce alcohol in excess of personal usage quantities the same could be done with pot. Age limits can be implemented and some of the new tax money could be used like it is with cigarettes for early education, not propaganda mind but actual education.
IMO prohibition of pot is akin to the prohibition of alcohol, and for the record organized crime syndicates that came to power from prohibition are still active, even though the FBI did land a major blow when they got Gotti and some of his associates. Go to Warwick RI if you doubt it. Ending prohibition didn't break Al Capone the IRS did. Jhowelb your point about a son or daughter turning to crime to support a habit isn't applicable to pot users. Pot isn't that expensive (cop friends as sources) and it doesn't cause withdrawals like "hard drugs" do that inspire junkies to crime. No one comes across as suggesting legalizing all drugs, which as you pointed out would be a disaster. But to address your point if it was legalized and my daughter started using it in the future I'd do my best to get her to see the reasons for stopping, but hopefully I will do my job as a parent so she wouldn't have a great deal of interest in it, or any drug, in this hypothetical future. A lot of these problems we face now in society could have been prevented by responsible parenting.
Just my opinion. That and 1.50 will buy you a small coffee so take it as you will.