The issue isn't so much that it can make the world a better place, anyone who thinks that the laid-back apathy that marijuana induces can make the world better is a fool. The problem is it's state as a Schedule 1 controlled substance. The punishment should fit the crime. More deaths and violence occur over the use of alcohol then has ever been recorded with cannabis use. You CANNOT overdose on weed, the quantity needed to body weight ratio is so high you would pass out from lack of oxygen long before you would reach lethal levels. Allergic reactions don't count as anyone with a gluten or barley allergy can suffer anaphylactic shock from drinking, say, beer. Same goes with peanut allergies or most other lethal allergies. If you're allergic you should obviously steer clear of it.
Smoking weed and then getting into a car accident is as sound an argument as saying that liquor should be banned because of drunk driving statistics. It's just plain silly. If your going to get inebriated somehow, you shouldn't place yourself in a situation where impaired judgment puts you or others at risk, that's just plain common sense. Same goes with smoking on the job. You wouldn't tolerate someone being drunk at work, why would you tolerate someone being high?
At the end of the day, the problem is that pot simply isn't harmful enough to justify it's current legislation. By legalizing it, but restricting it in the same way as alcohol (ie: only purchasable by adults, no smoking in public spaces, taxing it etc...), you would remove it as a source of funding from criminal organizations, free up a lot of prison space, save billions on enforcement and in fact generate billions of dollars through taxation. The fact remains that getting caught with weed in the US gets one slapped with possession charges. Being convicted of drug possession will prevent you from getting loans, jobs and generally fuck up your life as your automatically lumped in with coke dealers and the like. That simply unfair. It's not chemically addictive, you can't overdose from it and it's medicinal properties are well document. It safer than any over the counter pharmaceutical (which is often made from artificial compounds we don't fully understand that have laundry lists of side effects), it's cheap and easy to grow and at the end of the day, is generally harmless (or at least less harmful than much of what is already legal).
I'm not saying it's a miracle cure for everything, nor that it will change the world for the better, but it's current state within the legal system is oppressive for no good reason. Anything can be abused to the point of life altering effect. Food, booze, drugs, hell shopping addictions exist. It's not infallible, but it's not so harmful that I should be classified as a criminal and drug pusher for owning some and smoking it within the confines of my own home.