Some interesting posts in this thread. Just to dispel a few things..
Getting a prescription for marijuana is not quite as easy as you'd think. It depends entirely on your doctor and their personal beliefs. Those who view weed in a positive light are more likely to prescribe while those who view it strictly for it's medicinal purposes are much more strict.
And it's not like you can walk into the Dr's office ahead of time as ask about their policies towards weed.
Gov't does make some money from fines and tickets associated with "busting" people who possess. However, if they were to legalize it they'd make MUCH more in the long run because they'd have two major sources of Revenue.
1. The most obvious revenue is Taxes. Make it a legal product, contract companies to grow/harvest/sell according to gov't guidelines, and tax it. They could even go as far as to tax different strains at different rates, so you'll pay more tax on higher grade weed then you would on lower grades.
2. Busting illegal growers. Simply institute regulations on marijuana growing (think USDA for weed) and create a license that must be carried to sell. Those found growing out of regulation face severe fines as do those caught selling without a license. Now Joe Dealer either has to pay yearly for a license to sell or face possibly losing all his profits trying to sell something that people can easily go to the store for. Growers would now face losing money out of pocket in addition to their crops being taken and most likely used for the commercial product.
It would basically create too much of a financial risk for underground sales.
Then all you have to do is institute laws of use (i.e. Must be 21 yrs old; Can't drive under the influence; Can't use on public property... etc) and boom. Legal weed for the stoners and extra income for the Gov't.
It would also free up some space in the prisons by no longer having to imprison people for pot crimes.