What would be the advantage of using anything else but iBooks, especially if you have calibre on your computer and therefore can convert books easily to and from epub, mobi, and what else is all there?
Some people prefer iBooks' display; some don't.
Depends on the user and the content whether this matters, but as far as I know, iBooks DRM are the only ones that haven't been broken. So buying iBooks with DRM would mean sticking with Apple hardware or losing access to books.
I buy from Amazon
/Kindle because I want to maintain access to books I've paid for even if I decide to switch to hardware on another platform later. Barnes & Noble also allows access across platforms, but I think it more likely that Amazon
will stay in business than B&N. As it is, I can read my Kindle books on my Android and iOS devices, and on PC.
I've also backed up my Kindle books onto my computers, so that if I ever needed to gain access by breaking DRM, I could. I don't plan to do this unless something happened to unexpectedly cut off my access to books I've purchased, or if tech somehow evolves and makes my Kindle books obsolete years down the line. I would do this because I want access to my books for decades, till I die.