Probably not formally prescribe, but certainly many doctors have suggested to their patients that marijuana might be helpful for, among other things, reducing nausea and other side effects from chemo.
The real problem is that drug companies would much rather sell their own 'engineered' product for $100's/month than let a patient use something that could be grown for a tiny fraction of that cost.
Besides, while pot probably isn't as harmless as its proponents claim, the hysteria surrounding it is way overblown. People complain about "today's pot is 100 zillion times more potent than that of 40 years ago", but fail to realize that much of the push for more potent strains has been a result of the drug war. During prohibition, hard liquor was more popular than beer; today, the reverse is true.
The real "problem" with pot is that, were it legal, it could threaten many industries with unwanted competition. Not sure how to solve that 'problem', though...
Not really, you will find that the favorite choice of winos(addicts) is not a good Chardonnay, but cheap and very high alcohol content wines such as MD20/20 or Thunderbird.
The same would happen with pot.