Dispensing with the DRM debate: A PDF is not worth as much to the average consumer as a hardcopy. It just isn't.
Certainly, there are things you can do with a PDF that you can't do with a hardcopy... like simply copy-and-paste from it and drop it into your notes. And it's easier to carry around.
But it seems obvious to me, as someone who does use a laptop, that having a copy of a product on the computer, is considered an inferior alternative to having a book. If transport isn't an issue (for example, if I have it on my laptop and if I have it on a shelf next to me), it seems to me the book is preferable. I can't flip open to the page and hand it to a player to read while I am doing something else on my computer. Likewise, if I am in bed and I want to read up on something I have planned for the game tomorrow, and I have a book I can pick up an read, or if I have to dig my laptop out of its case and start it up, I'm going to grab the book.
Books are more pleasant to look at and more comfortable to use.
Compound this with the fact that most arguments about printing and distribution costs vanish, charging full cover price for a PDF seems pretty dubious.
Now, throw DRM into the equation, which introduces more hassles, the possibility of volatility, and portability problems, it becomes even more dubious.
Also, someone mentioned Malhavoc earlier. I do agree that PDF has some benefits. If its a product that I like and use a lot, I will buy more version. (And I was surprised in a poll I posted here how many people felt likewise.)