Very provocative what you say, and personally, I am annoyed as I disagree, but to respond, I'll try to think clearly when upset
First of all, many feelings are prohibited by laws, for instance the feeling of murder, rape, theft, and even suicide. Moreover, Euthanasia is amazingly illegal in most places. Walking naked in the street is illegal, and many other feelings are prohibited when you live in a society. Should all of them be illegal, obviously not, but many should be so we should talk about the specifics.
Similarly, the fact that no government is going to be able to completely regulate it is not an argument as it also applies to murder, rape, theft, suicide etc.
If drugs were completely personal and effect no one, like for instance gay relationships are, I would actually tend more to agree with you. But this is not the case. People on drugs harm society in many ways, and cost society a lot when they are being treated to cure them. This is also true for people who eat a lot, or smoke, but that's harder to regulate, though people are trying, with laws whose aim is to forbid you of at least harming others while smoking. Addicts don't only harm society when being cured but also in the day to day in different ways.
I can also add that I personally know several people whose life got completely messed up by grass. They had reasons to use it - they were very depressed, but it wrecked them.
I also know people who used different (light) drugs and really liked it, and felt it helped them relax and see different things as you describe.
This distinction brings me to another point: how strict should enforcement be?
In several southern states gay sex, or even sex between a white person and a black person, is illegal. I think in most states sex with animals is illegal. These laws are rarely enforced (though I seem to remember a case in texas a few years ago). Should medical marijuana be legal - definitely, should enforcement be very light - yes.
But why make it legal? Some people simply need protection. There are many laws enacted to protect people from themselves, and you can't trust people to always do what's best for them. Many laws protect kids, but should adults be thrown to the wolves? I'm not sure there is so much to gain from completely legalizing it, except some sort of regulation on it.
You must admit that the moment it will be legalized many more people would use it, and some people would use it more than at the moment. Moreover, they won't be as aware of its dangers as it's legal.
But there is a different argument and that of legalizing all drugs. In that case you lose a lot, but you also gain a lot by hurting the mob and the drug trade. At least in this case what you gain is enormous. I'm against it, but I think it at least makes more sense than simply legalizing marijuana which has a lot of very bad effects on the mind that people simply don't understand, and not that much will be gained.
There are other issues but I'll stop here.