It is, on available statistics, far less harmful than alcohol even when used in immoderation. No amount of marijuana will, for example, despite the best efforts of the Calif. Hwy Patrol, and others, to demonstrate otherwise, significantly impair the safety of a driver. In fact, in many of the tests they ran, the marijuana users were safer than non-users, because they became more deliberate and more in-the-moment than the control drivers--the upside of lowered short-term memory. You might ask yourself why alcohol is, according to the CHP, impairing above a .02 blood level, but no such measure exists for marijuana--CHP put a lot of expensive effort into establishing just such a number, and failed.
Although I agree, I would never advocate driving while under the influence of anything.