I'd be willing to bet that most people don't encounter horizontal scrolling on a regular basis. At least I sure hope not; it's hella annoying. The only cases where I can think of it coming up (for a full page, at least) is when there is an over-sized image or on the occasional artsy site where it's designed that way intentionally. Perhaps horizontal scrolling comes up more often for people using small screens--on a netbook or phone or something--but even then I imagine it's fairly rare.

The use case? Inline comments, feedback boxes, contact info, control panels, really anything you want to have immediately available but out of the way most of the time. Is this the best way to implement those? Not always, of course, but probably enough to warrant its existence.