Keith Humphreys believes the marijuana market is dominated by commercial grade pot because it's "an inexpensive product for a price-sensitive population." He claims that legalized marijuana would make highly potent, high-grade marijuana cheap enough for almost everyone:
The cannabis-using population would experience a vast increase in average drug potency. Caulkins and colleagues estimate that in the past 15 years, average potency of marijuana in the U.S. has doubled. But after legalization, with the 80% commercial grade market share being almost completely supplanted by sinsemilla, average potency would roughly triple very rapidly.
This increase in exposure to highly potent cannabis is one of the mechanisms through which legalization would result in a higher prevalence of addiction (Some of the other mechanisms are discussed here). It at first seems reasonable to assume that experienced users would simply titrate their dose of higher-potency pot, making higher or lower doses equivalent from a biological viewpoint. But surprisingly, laboratory studies of experienced marijuana users show that they are in fact poor at judging the potency of cannabis.
Kleiman is more cautious. He points out that "actual distribution of potencies would depend both on unknowns about consumer tastes, market structure, and branding/marketing strategies and on policy details yet to be specified."
The Dish 