They hand out those cards like candy so I wouldn't put much stock into the card. A very strict policy of being on the clock stoned and impairment during work would be in place. God forbid a terrible accident happened, they would be able to test to give an estimate on time of last 'toke.
Here is an excerpt from an article in Arizona.
the law prohibits discrimination on the basis of simply being a medical marijuana cardholder. In other words, having a medical marijuana card is not grounds to terminate an employee or refuse to hire an applicant.
the law does not require employers to allow employees to use, possess, or be impaired by marijuana at the work place or during work hours
Finally, for employers whose employees perform “safety sensitive” tasks, such as working with machinery or chemicals, working in health-care related fields, or driving vehicles, there are special protections, so it is a good idea to mention these safety sensitive jobs and clarify job duties and expectations in policies.
Seems you could argue for either side with all this grey area.