Consider the prospect of a state being completely surrounded by pot-legalizing states. Many people live along the borders. If you work in Kansas, and live in Colorado, you might go shopping at a pot dispensary and then leave your "package" in the car while you drive to work in Kansas. I can see where someone would bring up "constitutionality" arguments as to why they were arrested for merely possessing something that they could legally possess at their home.
And I think those arguments are good ones. As much as I abhor a state having draconian gun laws, I abhor even more the fact they could penalize a citizen carrying a firearm from one free state to another driving through.
the solution is almost always going to be less government. We are in more danger from too many laws than too few.
The danger of a few useless potheads, or even thousands of them, are far less than the danger of a much larger than otherwise necessary police force.