yes there is a reason to block legalization. Legalization will increase the number of overall users, there by increasing the number of impared drivers. It's not rocket science.
For that reason then you might as well block any sort of fatty foods since fat can cause a heart attack...thus anyone who eats anything with fat may kill someone if they have a heart attack behind the wheel.
Just because some people are stupid doesn't mean we need to make things illegal. Otherwise we need to make alcohol illegal, smoking illegal, and various other things as well.
I can't tell you the number of times I've seen an accident caused by someone not watching the road while lighting a cigarette. Heck I actually had someone hit me on my bike one time while doing that. Luckily they didn't hit me hard enough to hurt me, other than a torn pair of jeans and a badly skinned knee.
I made sure their hood felt my fist though...
The point being still police intoxicated drivers yes...but don't make something illegal just because it can lead to something bad...otherwise life has to be ended since it is the ultimate cause of bad things.