The mere fact that something is illegal to produce will tend to reduce the supply of it in most commercial channels. Yes, if someone really wants to download obscene materials, they will go ahead and purchase it online, and download it. You won't find it available as a "premium" channel when you check into a hotel, however, nor will it be offered by your cable provider. We can argue about whether this is a good thing or a bad thing, but the fact is that when you make something illegal, it removes it from "respectable" distribution channels.
You always have to ask the question: will removing a commodity from "respectable" channels make much of a difference with respect to the social problems that you seek to solve? Drug addicts will still seek out their drug, even if it is illegal. Most people will not seek it out, even if the drug is legal.
The people that are likely to be removed from the market for the drug are those who were not addicted, but were using it occasionally -- and they aren't usually the problem. The real gain may be the people that do not even start to use the drug because of its illegality, and who wish to avoid the stigma or the risk of purchasing through black market channels. I am not sure that the analogy of obscenity to drugs works very well. While there are people who are addicted to obscenity (in a psychological sense), I do not get the impression that they are the mass market for it. Making obscenity illegal means that a lot of people who occasionally watch the clearly illegal materials will find it more difficult to obtain. For many, this will be enough of a barrier to switch them to erotica that do not violate the Supreme Court's Miller test, or find some other source of entertainment.
