Personally, I prefer formats more flexible than PDFs, like plain text files.
DRM issues. Pricing issues. Platform issues. And, apparently, customer service issues. The ebook has a little more maturing to do, so for contemporary books I'd still stick with physical copies.
Despite that, PDF, especially PDF-A, is the superior format for preserving published content.
Reading plain text files is simpler to do across platforms. If an app on a mobile device just pays attention to the text then that's fine for functionality, I guess. But then I have a problem with the extra formatting being there, unused. Kind of like having a problem with unused CSS in a style sheet just sitting there.