Cannabis has medically beneficial properties; whether those benefits outweigh the harms of smoking for any particular patient is a question for him and his doctor, not government.
It has VERY limited benefits WHEN THC is extracted and used in actual medicine. And even then there are MORE effective drugs then any THC based drug out there. The whole idea of "medical" marijuana was and IS a sham designed to make the idea of smoking dope or getting high more palatable to a gullible public who can't see the propaganda effort they're launched.