It's not a straightforward answer but for me the option is always welcome - always. I can't think of single reasonable reason not to have one.
a) The expansion slot is a nod towards the future. A nod towards interoperability. A nod towards obsolescence. A nod towards uncertainty - somethings some groups have to be more aware of than others.
b) It's a nod towards people of variable income who might not always have access to a computer or the internet.
c) It's a nod towards sharing without a middle man. And many more things.
I wouldn't be quick to assume the success of Amazon has a lot to do with them understanding people.
As much as I like ebooks, they are one of the must kludgy techs this past decade and they barely move the needle in terms of capability vs actual books especially on e-ink devices. So far they are still mostly analogue transfers rather than a rethink of what is possible with current technology.
But the original point of the thread is true. I probably have too many books
