well i think you are asking about using one to get medicine??
instead of growing your own medicine??
their are nurmerous benefit's!
1.when you get busted you won't get busted for cultivation or manufacter just possesion
2.you won't have to put in a hard work taking care of plants
their are negative's
1.they are expensive
2.you have to go get them or let a stranger into your house to get them
hope that helps you out a bit if you need more just more specific question

also read your state's MMJ law's prop215 compassionate use act of 1996
California SB 420, 2003
SB 420 Senate Bill - CHAPTERED
SB 420 Sets Prop 215 Guidelines
California Proposition 215 (1996 - Wikipedia, the free encyclopedia)

there's a few links to get ya going