Well, all legislation deals with morality; I think your (completely valid, IMHO, incidentally) concern has more to do with its granularity. Proving harm is tough, which is why I think it's an excellent test, as you say: if you're having a tough time making your case, maybe you should consider dropping it.

Now, on your examples, I think you may be taking it a bit too far. Specifically, if your possession (and, presumably, your use) of some illegal product promotes or encourages its trafficking, I think you could effectively argue that it too should be illegal. Now, that doesn't mean that you expend the same resources in enforcing these laws: I don't think anyone would argue that busting everyone with a dimebag or picture or two is an efficient use of resources. However, if I'm spending a lot of money consuming an illegal product (be it child porn, illegal drugs, etc.), I think it's reasonable to say that I'm contributing to the problem.

But again, it's the level of granularity that's key: it was eventually decided that prohibition was excessive. What about marijuana? Polygamy? I don't know the answers to these questions, but I do feel that intelligent debates concerning provable harm would be an excellent starting point.