There was a lot more information in that study link that seems to lower the study value to near zero.
A sample size of only 14 with a control of 14 picked from a drug treatment program where there was others known factors including alcohol abused and in that it was a group from a drug treatment program therefore the likelihood of unknown others drugs used seem very high indeed.
Pick a few hundred marijuana smokers who are not in drug programs and compare them to non-smokers and then do a brain scan and you might had some worthwhile results that have some meaning.
In the current study, working with child psychiatrist Sanjiv Kumra, M.D., now at the University of Minnesota, Ashtari and colleagues performed imaging studies on 14 young men from a residential drug treatment center in New York State, as well as 14 age-matched healthy controls. All the study subjects were males, with an average age of 19. The researchers performed the imaging studies at Long Island Jewish Medical Center.
The 14 subjects from the drug treatment center all had a history of heavy cannabis use during adolescence. On average, they had smoked marijuana from age 13 till age 18 or 19, and reported smoking nearly 6 marijuana joints daily in the final year before they stopped using the drug.
Ashtari added that the findings are preliminary. Among other limitations of the study, such as a small sample size, five of the 14 subjects with heavy cannabis use also had a history of alcohol abuse, which may have contributed an effect. Also, it is possible that the brain abnormalities may have predisposed the subjects to drug dependence, rather than drug usage causing the brain abnormalities.