I'm late to this conversation, but here's my .02 (for the record, I have never indulged):
1. Dozens of things can impair a driver. It is the responsibility of the driver to make sure he/she does not drive while impaired. Also, the rationale that legalizing pot will result in a huge spike of safety problems is knee-jerk, at best.
2. IMO, those who want to use already do, for the most part. Legalizing it would actually make it more difficult for minors to get, and offer greater incentive to adults to use it properly.
3. Instead of flushing bazillions of dollars away chasing the occasional bale of mexican weed (read: War on Drugs), we could be reaping the revenue generated by the proper management.
4. The concept that adults can be trusted with alcohol, but not with marijuana is laughable.
5. If there is such a thing as a "gateway drug", alcohol is it.
Again, I haven't ever used the stuff, and I don't ever plan on doing it. But I'm watching my mother-in-law slowly deteriorate with MS, and the meds they have her on are doing as much harm as good. Side-effects all over the place. If she could use marijuana instead, a lot of her discomfort could be dealt with, w/o the ******* side-effects.
Pot suffers from an archaic stigma - a government campaign from the 30's that painted it as the gateway to insanity.
I apologize if any of this has already been covered.