Is there a reason why I keep getting moderated down here?
I downvoted you because Viliam claimed medical marijuana prevents a great deal of pain, and you responded with - an argument that fails to even assert it causes pain elsewhere. Perhaps you could start by showing that what you call the "central" use of medical marijuana does not displace other recreational drugs.
Over at Scott Adams' Blog you can find a very fine example of using the 'Rationality Engine' to solve the social problem of assisted dying.