Tell that to the folks killed in traffic collisions by stoned drivers.
Sir Gawain's post was in rebuttal to the ridiculous argument that marijuana is more harmful to the user than alcohol. As far as indirect harm to third parties, alcohol is #1 by a wide margin (not to mention the immorality of punishing ALL users of a substance for what SOME users do).
I'm not advocating driving stoned. There is already a law in place called DUI/DWI that covers any mind altering substance. You can get a DUI from too much OTC medicine also ya know.