Isn't that what Marinol is/was?
I tried Marinol once (just a little science experiment - I swear). It's great stuff if you don't mind the fact that it leaves you unable to carry on a conversation.
The advantageous thing about actual marijuana as a pain-killer, etc is that you can control the dosage. With Marinol it was take the whole pill or nothing as it was a little round "pill" with yellowish goo inside i.e. you couldn't split it up.
But wtf is the point? There's already a way to simply "synthesize" the active substance, and at virtually no cost. Why would I want to pay hundreds of dollars for some synthetic product when I can simply grow the real thing in my back yard? Let's see... pain relief plus altered mood with nearly no cost, available immediately, vs. pain relief plus unknown side effects for hundreds of dollars, available at some unknown time in the future. Tough call, eh?