Marijuana is not a benign drug. Use of this drug impairs learning and judgment and may lead to the development of mental health problems
etc. etc.
Just about the same arguments can be made for alcohol.
As for the medical marijuana debate, every rational person can see the difference between street use of opiates and use under doctor's supervision, whether it is codeine, morphine or the many synthetics. One of my old coworkers had nasal surgery and was given cocaine as one of the drugs before/during surgery. What is it about the "demon weed" which incites so much fear and hatred that it might have a medical use?
Considering that alcohol has non-intoxicating uses, its legaility really has nothing to do with the question of allowing marijuana to be used for medical purposes.