That's a very important distinction.
Contrary to music, I consume my ebooks on one device: my e-reader. My laptop, phone and tablet are terrible for reading. Why would I need my ebooks on my phone or tablet.
The only thing I do want, is to process my annotations on my computer. Which calibre cannot do (but a python and bash hack does provide me).
For reference materials, textbooks, comics, and other image heavy and laid out precisely books, I almost exclusively prefer PDFs and only read them on my tablet.
If it's a book with almost zero images, like most fiction, how they're laid out doesn't matter, and I enjoy the ability to read anywhere on any device. I've never found the need for an e reader and regularly switch between my tablet or phone. Reading comics on the phone isn't for me, but there are a lot of people that don't mind it.
For me it's basically a way to use two plugins and occasionally fix a book cover. It's a bit bloated IMO.