The problem is alcohol is black and white. Their are scientific limits to what you can drink, and cut and dry impairment tests. How do you know define when someone is too stoned? Regardless of why you use marijuana (recreationally or medically), the effects are the same. Just because someone has cancer or whatever doesn't mean weed won't get them high.
It's definitely a tricky situation. I could personally convince myself of going either way with my opinion. Same with prescription pain killers and other mood altering pharmaceuticals.
I would have to imagine that if the gov't is looking to leagalize and regulate marijuana (which I support), then jobs can be created. There will be a demand for scientist to do studies and determine a way to help LEO with impairment tests, etc. When the legislative branch of gov't decided to put a BAC level restriction on those who are driving, someone had to figure out a way to determine the BAC of said drivers. Why can't someone come up with a way to determine the level of "B'THC'C" with a simple device just like the officers' breathalizer tests for alcohol? Obviously, it would be a different system all together but the same principal would apply. You get pulled over and the officer suspects you have been ingesting THC, you get your finger pricked with the device and your level is displayed right then and there. Someone has to invent the hypothetical "finger prick tester", someone has to mass produce it, someone else has to come up with a better unit........