And this lies more on a feel thing: I feel that when you are looking at images of the printed page (effectively), you are looking at something relatively legitimate, I know it's an exact copy of the actual book, and that conveys with it a much more solid sense to me than looking at what might as well be a text file someone sent you.

That being said, features like table of contents links, search, being able to copy/paste from the material are huge. I probably wouldn't mind if they figured out a way to let me look at a page as a PDF and then "zoom in" on reflowable text on my phone or something. But to me, the material being presented as intended for print is pretty key, otherwise you might as well just be on someone's website.