I just find the axiom "drugs are bad" to be more or less indefensible.
I think we can all agree that some drugs (no matter how you define the word "drug") are beneficial.
If instead the assertion is "illegal drugs are bad", you leave yourself open to some pretty glaring contradictions, as most illegal drugs also have medicinal properties and are widely used as medicines (opiates being a perfect example).
That really only leaves you with "abuse of drugs is bad", which I (and I assume just about everyone else) have no problem with.
Abuse of cannabis is bad (yes, a tautological statement). Its psychotropic properties, long-term and short-term side effects, and medicinal properties should be considered carefully when making decisions about use (just as with all drugs...and, well, anything you consume/do). With that said, many people receive benefit from its use, and unless the damage it causes to the individual or his community/society is greater than the benefit, I see no reason why it should be prohibited entirely.
Not all people who smoke weed are stupid stoners.
On a personal note, I could probably get a prescription for cannabis for an assortment of ailments, but choose not to. I regulate my use of cannabis (I haven't smoked since last Wednesday), inform myself about the negative consequences of use, and try to consume it safely (that is, I don't smoke and drive, or pursue other potentially harmful activities while I'm high). I see no reason why I should feel ashamed of this behavior, or otherwise feel compelled to discontinue use.