Earlier quoted context omitted.
If you've got a kid, especially a young one, 16GB isn't enough for pictures and video. Not even close. My niece has filled her 64GB iphone many times over with shots of her kid. I'm sure there are tons of other use cases too.
Secret feature : plug your phone on a computer via USB and transfer your pictures.
1) The assumption that the person who owns the phone owns a computer, rather than borrows someone else's when they need it.
Modern phones have hdmi outputs, and work with bluetooth keyboards. It is entirely feasible for the average person to use a high-end phone as their primary computing device. Especially with the carrier subsidies making the up-front cost so low. This will only become more common as phones get more powerful.
2) When you've moved the data off your phone it is no longer immediately at hand.
Flash storage is cheap and only getting cheaper, leaving off a micro-sd slot is all about pushing a cloud-storage paradigm rather than letting the user decide.