I often use this trick in document readers when I want to refer to something else for a moment and then return to where I was; e.g. I can scroll down to an endnote, or scroll up to the table of contents, check something out, then drag sideways and let go and I'm back where I was and I continue reading.
Unfortunately, at some point people started making applications where scrolling only displayed page numbers, not displayed the page (e.g. Word, although I believe it can be configured to the old behavior). (Some are even worse, and display nothing.) The latest culprit: Adobe Reader 8, which is showing me (unreadable) thumbnails when I drag, and which doesn't seem to have a way to configure it to the useful behavior.
Why does this happen? All I can think is full page scrolling is too slow, so they have to fall back to this, but computers are getting faster, not slower.