The first it would seem is the assumption on the part of WotC is that the piracy occurs because they made the product available digitally. However my experience is that the product will become so no matter if WotC makes it digital or not. Given that I have pretty good evidence that piracy occurs way up their logistical train I think it is safe to say that PDFs or no PDFs WotC will get pirated.
I think the second flaw in the logic is that reducing piracy will increase sales. It only increases sales when the customer has a legitimate way to obtain the product in the new format. That is the problem that the recording industry faced. The record companies were so asleep at the wheel that they did not see the MP3 coming and it took them about 2 years to find out what happened to their CD sales. Essentially CDs had become obsolete and there was no way to obtain MP3s legally. Now there are outlets like i-tunes and this has given legitimate buyers a place to obtain music in the latest format. The paper world is just slower. CD to MP3 happened fast because it was digital to digital. Publishing is lagging a little because it still is largely paper to digital. Publishing has gone digital on the production side but not entirely on the distribution side. Thus it is easy for publishers to make a digital product but their logistics are such that they are still tied to paper for profit.
Though what I suspect has happened is that WotC has decided that they actually can do better at electronic sales if they are the sole source. After all they have the records of the sales and know the numbers and how much they a loosing to the middle men One Bookshelf and Paizo. Suing the people that distributed PHB II might be part of the same move for a different reason.
In conclusion I think that WotC will loose sales from this move until they have a way for people to obtain their content legitimately. Until then I suspect that electronic piracy of their products will undoubtedly increase.