It was front-page news early this year that doctors nationwide still could be prosecuted for recommending marijuana. A class action suit just filed in California tries to protect doctors' free speech and their licenses in such cases. Medicalizing otherwise illegal drugs depends on efficacy and safety data, civil rights and politics. It also, in managed care, depends on the formulary. There is little efficacy data, but lots of grandstanding and loads of politics on this issue.
Dronabinol seems slow to stimulate the munchies and control nausea, some patients report.
It is weaker than marijuana, it is expensive, and it is not commonly prescribed. Marijuana, on the other hand, poses the risk of aspergillus pulmonary infection for the immunosuppressed. There is little hard scientific data readily available to compare dronabinol and marijuana for the same indications, side by side. But different drugs act differently for different patients. After all, the most effective drugs for pain are narcotics.