Here is my (admittedly out-on-a-limb) example of a reason to use Repeat One and change the Prev./Next behavior.
I listen to a lot of video game music, and there are occasions when I like to use it as background music (for example, while doing a particular (and longer) step of a chore or job, and I like to get a similar feel to a game by putting a VG track on Repeat 1. (Please don't ask me why, it's hard to explain.)
But even I can get bored of the same track after a while, and so I queue up a selection of tracks that I think are fitting for the events at hand. But I don't want to (maybe even can't) stop what I'm doing for a few seconds just to change the track. For this reason, I reach - or at least, I would reach - for the Next button.
One slightly saner example might be a person with a new album who likes to learn the lyrics of every track in succession - or simply wants to savor each new song a few times and then move on. The easiest way to accomplish this would be to queue the album and turn Repeat One on, then hit Next when you're ready to move on.
Yes, I know, oddball examples, but examples nonetheless.
