Legalizing for medicinal purposes does not necessarily imply making it free for everyone to grow and use. I think that marijuana should be treated as any other medicine - administered by subscription only, as pills, tinctures, injection, infusion (as tea) or whatever would be a good method for ingesting the active substances, but not as something that can be smoked.
I am strongly against smoking - of any drug, because smoking is an effective way of spreading drug-addiction to people who don't want to become addicts -- which includes everyone who isn't an addict already.
In the case of smoking tobacco, no tobacco smoker has chosen to become a smoker. There are those who say that they have chosen, but they don't know better, because they are addicts. Practically all tobacco addicts are addicts because they had been subjected to enough second-hand smoke to trigger the addiction.
Smoking is also a stupid way of ingesting drugs, because it also produces many harmful chemicals such as carbon monoxide and so called "tar" - which consists of literally thousands of chemicals (slow-burning a chemical does not combust the chemical properly, but turns into many variants between uncombusted and combusted, and these half-combusted chemicals can be more reactive and therefore dangerous).
People who want to legalize marijuana (for everyone) often compare it to tobacco, and say that if tobacco is free to use then marijuana should also be.
They don't see that the legality of tobacco is a special case: Tobacco is legal because of pressure from tobacco companies but mostly because the number of addicts is significant enough. If it's use had not become popular early so that there was a large amount of addicts at the beginning of the 20th century, then it would also have been made illegal at about that time, and we may now have discussed tobacco instead of marijuana.
That said, I think that the US is sentencing people too harshly overall. Possession of a small amount of marijuana without a license or medical cause should give you a fine. Maybe a short prison term if you are caught with the third time, or something like that.