Well, I disagree about the more benign than most other pharmaceuticals part, and hydrocodone is just another opioid that isn't actually that physically harmful if you don't become an addict. Sure lots of the opioids in use are modified/made in a lab, but that doesn't necessarily make them bad. There's a reason most people grab an aspirin for a headache instead of finding themselves a willow tree.
My opinion on part of the reason that pot isn't as accepted is that most of the medicinal uses I've read about give inconsitent results. There's also some issues with dosing and administration... Oral absorption is pretty variable as I understand it, and if you're using the plant rather than an isolated compond, standardizing an amount is pretty tricky. You're also not going to see a lot of doctors that are happy writing up a prescription for someone to smoke anything (yeah I know, vaporizers... ). I know its not as fun as a conspiracy, but there's actually some legitimate reasons as to why its not really an ideal drug for many problems.
That being said, I think that its not that harmful in most cases, but that its less "benign" than the legalization side makes it out to be, even if we're not talking about smoking. I'd be more supportive of research to modify cannabinoids into a more easily used form and understand their effects a bit better than I would be of broad legalization.
Of course, I'm probably more than a little biased and it has admittedly been a few years since I've cared enough or had the time to read much on the topic.