The consequences of smoking weed for physicians can go far beyond that of just being fired. I've seen a couple of cases as I've traveled around the country of employees being caught. While point one may be be true, remember that there is no federal law banning people from having beers on a night off while, for better or worse, smoking marijuana is in direct violation of federal law.
So in certain states doctors can loose their licenses (KY, TN, UT come to mind). In other doctors are suspended (MA, NY, CT come to mind). In all cases repeat offenders can end up in jail (there is a doctor that had her licensed revoked and is serving a jail sentence, in MS, for this as it was in direct violation of the contract he signed with the hospital and in violation of federal law).
I thinking "risking" it is not worth it if/until there is a change in federal law. You pretty much have everything to loose (medical license, career, reputation) and the potential to face time in jail, while there is very little to gain.
Although, I don't smoke, I'm all for the legalization of marijuana. However, as long as it is illegal, you are better of refraining while a practicing physician.