i don't have a problem with medicinal use of MJ, but....since i live in a medical marijuana state, i have seen first hand how well this doesn't work.
1st, you have the issue of acquisition. oregon overcame this by allowing a person to grow 2 or 3 plants or designate a grower. every time they bust a place growing 100's of plants, the grower claims to be growing it for others :-)
next you have the problem of standardization. we all know there is "good" pot and junk stuff like ditch weed. a patient would not know, even growing their own plants, the dose from use to use. a solution is to extract THC and use that, but that takes all the fun out of it...and they say it doesn't work as well.
3rd, pot smoke is very hard on the lungs. in some comparisons between pot and cigarettes, pot has been found to do more damage. of course, you don't smoke as much pot (we hope) as cigarettes, but still..who pays for the lung damage to the person who smokes the pot. chances are, if they are using it medicinally, they are not working.
what happens in the work place to drug screens? with HIPAA rules, you can't ask if a person is using a drug legally. you can only fire them for popping positive on a legal drug if they are impaired, or if you can prove that any use of the drug puts the person or co-workers in danger. you can fire someone for being drunk on the job. we know that alcohol stays in the system for a short period of time, so if a worker has alcohol in their system, we know there is a problem.
one of the biggest problems with people who routinly smoke pot is that they become very apathetic. any number of studies have been done, and we all probably can remember someone who didn't finish school, or blew off friends just because they didn't care any more.
we already have a problem with young people and alcohol use and abuse. it's hard to tell a kid they shouldn't drink when they know it suddenly becomes legal after age 21 and they see dad with a beer every night. seems it would be equally hard to convince kids that they shouldn't do drugs when they know it will suddenly be legal at age 21 and they see mom or dad with pot growing in the garden.
all that said, i'm not sure that it should be a priority of law enforcement to go after people who have enough pot for personal use. if they are dealers, or are DUI, they should be busted. for others, a fine and/or some kind of misdamenor record for the first bust or two would be good enough. after that, you might find that you have to do something a little more harsh to get folks attention. 30 days as volunteer with teens would do it for me.!! :-)