Pot should never be legalized and I hope that the federal government goes and shuts down all those quacks, and I will tell you why, outside of the fact that it lives up to its reputation that its a gateway drug.
Marijuana levels in the body do not correlate with levels in the brain and especially do not correlate with a nice dose-response relationship like alcohol does. What is this mean, well take the example of alcohol, you get a BAC from a breathalizer, you know with a deal of certainty what the concentration is circulating in the body and you typically know what kind of response you would get from a person at a given BAC level. Sure, there is a little variation but for the most part its true. Little ol' Mary Jay on the other hand doesn't have the predictability that alcohol has. You don't know why concentration is circulating in the brain and everyone's response is different at different thresholds. Meaning you there is a great deal of variability and its difficult to police. Not to mention I can't even imagine a bunch of drivers running things over without a care in the world.
And then theres the issue of fat distribution. In heavy smokers and those who are fat, hash distributes in the fat tissue. Urine tox screens can be positive for months after stopping use. I once saw a person test positive for 6 months after claiming to stop use with a degree of certainty in her convictions (in other words it was scientifically feasible).
So I feel wholeheartedly that marijuana should be banned and users should be persecuted. We have marinol out there and Sativex is on its way. That should be good enough. Medicine has come a long way in 10 years.