"If you are for legalization, do you want your surgeon to be high while he operates on you? Or your pilot to be high while he's flying the plane? Or your bus driver, child's teacher, etc."
The rhetorical question makes many unsupported assumptions:
Assumption 1: Prohibition prevents people from getting high.
Prohibition fails to prevent people from getting high, so the odds are the same my surgeon will be high with or without legalization. Any decrease in drug use due to prohibition is negligible.
Assumption 2: A substance that is made legal will, as a result, be used by everyone.
Currently many thousands of substances are legal and on the market, yet everyone is not "on" them all the time. Though alcohol has been legal since the mid 1930's, there are many millions who manage to avoid being drunk as they do their job. Ditto for pharmaceuticals, all of which have side effects.
Assumption 3: Using cannabis prevents people from doing their job well, or even adequately.
There is just no evidence of that when it comes to the vast majority of jobs. Naturally, no one wants a service worker whom they rely on to be impaired, on anything! It makes no difference if the impairing substance or activity is legal or illegal; cannabis, or modeling glue.
Assumption 4: Cannabis couldn't possibly benefit anybody's health or work.
Do I want my surgeon to have used cannabis before operating on me? Well, let's see,... is he epileptic and prone to seizures if he doesn't use cannabis? Then, absolutely, YES, I unequivocally want him to use cannabis before he operates. Why, would you want your surgeon to skip his vital anti-seizure medication before operating on you? That seems a bit, well, reckless.
That's just one scenario, but others come to mind: muscle spasms, chronic anxiety, depression. Any of these might negatively affect a surgeon or pilot's performance, so if these conditions can be mitigated by the use of cannabis, and without any serious side effects, then that is clearly a positive.
In the end, this argument boils down to the same old simplistic moralistic screed: "drugs are bad, mm'kay?"