A few thoughts from a different angle.
1. It's considered an illegal substance by the United Nations. Now I am not big on the UN, but it is a world body that is trying to deal with this stuff.
2. I am not aware of any methods that the police can use to detect marijuana impairedness. An equivelant to the breathalizer. People shouldn't be driving, flying, operating, teaching, etc stoned. And there is no legal method to determine this short of blood tests or other invasive tests that can be refused by the subject.
And what about combinations of drugs. .08 alchohol + how much THC means impairment?
3. Some countries shoot people for selling a joint. Yet we are telling our people its ok to do that. Big big difference here. On our side of the line do whatever you want. On their side of the line you get shot in a firing line. Do you think any of our little stoners will ever take a holiday and forget this little point?
4. Crossing the border into the US. They take this stuff pretty seriously down there. If they find one seed in your car they can confiscate your car and any other property they feel like and toss you in jail. Ask Tommy Chong about that some time. I can see the day that the stoner gets out of the American jail and then procedes to sue the Canadian government for $400 million for not protecting him or some other equally stupid reason.
5. How can you make end user consumption legal but leave cultivation/importing/distribution illegal? It makes no sense. You're just giving an incentive for people to perform illegal acts. If you just make it all legal then the consequences could be very dire. Every criminal organization in the world would move to Canada and make it their base of operations.
6. Quality control. I've seen all sorts of sick stuff being laced into marijuana. How do you know what exactly is in that reefer? By having the authorities condone the action or legalize it, what sort of liability is the government incurring if some poor stoner gets his brain fried?
So the government ends up regulating it and you end up with the garbage being grown in Flin Flon that nobody wants anyways.