As one data point, I explicitly did not purchase an HTC One because it did not have an accessible SD card. It is a key buying feature for me.

The main reason I like the external SD card is that I can bump up the storage capacity tremendously, which has all kinds of benefits. It also has other nice perks: changing between devices becomes easier, easier to use as a portable USB drive, etc.

I feel I cannot be alone in this, but I don't have any hard numbers for you, sorry.