I can’t speak for DPW’s reasons, but based on my observations and experiences, there are some people for whom the occasional hit of weed is not much different than drinking an occasional beer. Like alcohol, it’s very destructive in excess, and some people shouldn’t do it at all. For instance, I used to smoke weed when I was a teenager, and just a couple of hits would make me pass out and drool on myself. 
But I knew people who could smoke occasionally and in moderation without being noticibly impaired… so I wouldn’t dogmatically say “it’s always bad,” any more than I’d damn somebody for having an occasional drink.
Incidentally, I’ve known more than a few people who occasionally used cocaine as a social recreation – again, in relatively small doses, like having an occasional drink. I’d go so far as to say that all things considered, provided it’s used in moderation, cocaine is a less destructive drug than weed. I’d be more likely to get in a car with somebody who had just done a bump of coke than with somebody who had just taken a hit of weed or had a beer, for instance.
I can’t think of any other drugs which I would consider possibly appropriate as occasional recreation, other than alcohol, weed, and coke. And, by the way, I’d have to spend a day qualifying what I mean here, so just consider my comments to apply only to the “best case scenario” – that is, a person who is thoroughly responsible about his actions.
As for the morality of drug dealers, there are SO many compounding factors that I can hardly answer the question. Drug sales are illegal, and most of the people at the top of the line are hardcore violent criminals. So by involving oneself in that market, one is supporting that sort of violence, which is of course not moral. Most drug dealers are also rather unprincipled about their sales. A bartender, for instance, will cut someone off when it becomes apparent that he’s had too much to drink; a typical drug dealer will never turn down a customer, unless he thinks there’s a security risk involved. To this end, though, it might be worth talking about the responsibilities of a bartender. If a bartender has a customer who comes in every night and gets totally smashed, does he have a responsibility to eventually tell the customer not to come back until he gets his life together? There’s also the fact that marijuana has valid medical purposes, which means that whatever else you think about it, there’s at least one clear case in which it is thoroughly moral to provide someone with access to weed.
I don’t think it’s possible for someone to morally sell cocaine in the current legal system, though I’m open to argument. I can certainly see a case where it’d be moral to sell weed: for instance, if one grows one’s own marijuana and sells it to moderate recreational or medical users. (I’m putting aside the possibility of being arrested, which seriously complicates the issue – it probably means, I think, that one would have to have a serious personal value at stake, such as a loved one who needed the weed for medical reasons.) If drugs were legalized, though, I think the possibility of it being a moral market could be much greater. Any thoughts?