"Law makers seem to consistently get it wrong. Why should we want regulation or believe that government will do the right thing?"
Arguments for state regulation that could be considered positive would be to determine if growers / product manufacturers are conducting their businesses ethically and if their products are what they say they are in terms of purity and potency.
Concerns for regulation include the possibility of de-privatization of marijuana growing by patients and excessive taxation / fees at all levels of the supply change as well as at the patient level. Already exorbitant marijuana prices will likely sky rocket. These factors may make it more difficult for patients to acquire their medicine, unless insurers become involved (which will naturally drive up the already exorbitant cost of medical insurance).