From the point of view of a neurobiologist who has studied the affects of drugs on the brain, I have a couple of points I's like to add to the discussion.
1) Absolute abstinence is, of course, the healthiest option.
2) There is a subset of the population that is afflicted by a) severe anxiety disorders (such as PTSD or related Borderline Personality Disorder, as examples), b) debilitating insomnia, c) very poor apatite due to the above, diseases such as Chron's, or very commonly due to side affects common with prescription drugs such as chemo agents, ect.
3) For this subset the use of marijuana can and does provide clinically relevant symptom relief when used in MODERATION. Again, for healthy individuals, absolute abstinence IS the healthiest choice.
4) Expanding on the above, for individuals with genuine mental and/or physical syndromes, marijuana can provide very effective relief when compared to current alternatives. For anxiety disorders it can be more effective than benzos such as Klonopin, Xanax, ect, without the severe effects on individuals and society at large these drugs tend to have due to their high propensity for addiction, subsequent abuse, and death (especially when mixed with alcohol). For apatite disorders, marajuana is quite simply the best antiemetic yet discovered.
5) Marijuana, like any other psychoactive drug, can be disruptive to normal brain development, which concerning the frontal cortex (that which makes us sentient humans) occurs from prenatal development through the early 20s. A little longer for males than females, in general like most development.
6) All drugs have both "intended" effects, from the medical point of view, and side effects. Marijuana is no exception. Its use does have serious drawbacks, not limited to memory disruption, psychological addiction, and pernicious apathy.
7) to address the link between marijuana and violence discussed here: correlation does not prove causality. This is a basic tenet of logic and science. My individual opinion is that any violence associated with marijuana use and distribution is due to its illegality, and said violence is caused by its relegation to the black market in the vast majority of nations, not due to any direct effects of the drug itself on the brain (it likely has the opposite effect. see: the amygdala).
Finally 8) again, an individual opinion: we as a modern society should accept that a certain subset of the population has and always will turn to drugs, prescription or otherwise, for a variety of reasons. Enforced complete prohibition has never been, and will never be, a realistic goal. Therefore we should focus our efforts on controlling those substances which have the most likelihood to harm the individual user and society at large. I'll leave it to you to decide whether marijuana fits this description.