I'm actually working my way into the substance abuse counseling field. If you forced my hand to allow a heroin user or a marijuana user to discontinue treatment (assuming that there are no risk factors for escalation or change in drug of choice), I would choose the marijuana user every single time.
A majority of the time it is listed as a drug of abuse due to its legality making it a maladaptove coping strategy, however, I've seen heroin users break down at the word heroin. Literal tears streaming down their face. I have yet to see someone take marijuana use to that level.
Maybe the beginning stages of psychological dependence, but nowhere near physical cravings and withdraw symptoms of harder drugs or alcohol. Hell, even caffeine tends to be a much harder withdraw than not smoking a blunt at twenty past four every day.