The only use for alcohol, cigarettes, and caffeine is also the high. The mind-altering effect it has.
There is nothing inherently illegal about having a good time and a drug doesn't have to have positive health benefits to be legal. Cigarettes have no health benefits and the effects of smoking them are even worse on your body. They're still legal because there's no good reason for them not to be. They're still regulated because there is good reason for them to be. Marijuana's no different.
The government simply has no right to send people to jail for doing something to their own bodies in their own homes that harms no one else.
Marijuana has been stigmatized due to a successful campaign by William Randolph Hearst and Henry Anslinger, who we now know prohibited it to get rid of competition for his newspaper business and painted it as a moral issue to get support from the puritanicals who would also ban lots of personal, private choices one should be free to make with their own body.
Marijuana is in no way substantially different than alcohol and cigarettes, the government's own commission intended to probe deeply into the legal status of marijuana concluded the same thing.
By the way, one of the major difference between Marinol and smoked Marijuana is that one negatively effects your liver and the other negatively effects your lungs. For people like my aunt (an evangelical who has never even been drunk) who battled cancer for years and took every prescribed drug in the book to treat and deal with the pain, their livers have been completely destroyed by pills. Once your liver goes, you face some massive problems and getting a transplant is a risky crapshoot (also, sadly but understandably, not one a terminal patient is likely to receive). Marijuana does damage your lungs, but if you've got pink lungs and a couple years at best, it's a safer alternative to any oral medication. Marinol may be better for some, smoked marijuana better for others. But both have positive effects and negative side effects.