THC is only one cannabinoid. Cannabidiol is a non-psychoactive cannabinoid that has great medicinal purposes.
It's been proven that mice and rats won't self-medicate THC, I don't know if I've said that, here, before, but I figure we have a little more willpower, and intelligence, than a rat. Or mouse.
I looked up Marinol and found this:
Many users complain that not only is the cost of Marinol greater than the cost of the amount of marijuana needed to produce the same effect, but it is also lacking in many of the properties of the whole plant, which contains hundreds of different cannabinoids. However, Marinol is available for free to those who can demonstrate their financial need to Roxane and Unimed Pharmaceuticals. In addition, it takes over an hour for Marinol to reach full effect, but it's duration is longer and it is available in controlled dosages, avoiding the uncontrolled strengths that occur when smoking marijuana. It is difficult to maintain an exact level of effect (which is important to most medical users), as well as making its use difficult for people suffering from nausea and vomiting, since they often expel the medication before the it takes effect.
http://encyclopedia.thefreedictionary.com/Marinol