I don't know if your question was directed towards me Coolio, but most people that I know in CA with medical cards do buy it, as they are entirely clueless to the growing process or simply do not have the resources to do so (dirt, water, and light).
Even for people who grow their own, the dispensaries are always worth a stop for the other things they sell - novelties, hash, edibles, t-shirts, what-have-you. I never grew my own but a few of my friends did, so thats just as good (easier, actually), so most of the time it came from them, but I would still occasionally stop at dispensaries for random shit.
Anyway, of course I agree, anyone who can grow their own should. My area allowed for 12 plants, 6 mature, but many allow as many as 99 plants, and in some circumstances, there is no limit (I don't know how, but I've seen it, and it was legal). Buying when you can grow your own legally is just burning cash.