Pretty much all the arguments for keeping weed criminalized are BS. one that might hold some credence is that it would be easier for minors to get a hold of it, but it's not even really true and if it were legalized they could regulate it properly. I think driving while high should probably still be criminal (depending on how high you are, although this raises the issue of how to test for it accurately) but smoking it itself is fairly certainly less harmful than a same-level consumption of cigarettes or alcohol.
The amount of wasted resources by the government on weed dealers is absurd.
At the very least, medical marijuana should be available everywhere.. it provides legitimate relief to a great number of people, and does so more adequately and with less downsides than do other medicines.