As with every known prescription drug there are those that will always abuse them.
Most people who use prescription drugs do so as doctors order them and no more. We cannot condemn any program of public assistance because a few people find a way to exploit the system.
This problem is inherent in any government program, but is way more prevalent in programs giving relief, tax breaks, subsidies, kickbacks, etc... to the wealthy than what is exploited by the few who actually need the assistance.
There is absolutely no logical reason for NOT legitimizing the use of Marijuana as there is no evidence that this is actually a gateway drug to hardcore addiction.
Keeping this drug illegal makes as much sense as banning all legal prescriptions because some people "might" abuse that drug.