"you can't set a right margin for a block quote"
You were posting before the Fire was widely released, of course. KF8 devices and apps do support a right margin indent.
For those of us who make a living by writing and publishing, the Amazon formats are all-important. In my experience since Barnes & Noble began selling ebooks, Kindle editions outsell Nook editions ten to one, and none of the other retailers are anywhere near B&N.
And that's only on Amazon.com! The UK store also outsells B&N by two or three to one.
I am sure that others' experience is different, but I have been actively publishing ebooks for nearly five years, and that is how the business has worked for me.