Correct me if I'm wrong, but I thought the original idea of impostors was to use dynamic billboards for distant objects, and regenerate them on the fly as the angle or distance changed enough to make the error visible? Although this certainly still qualifies in the basic sense of "render a simple thing that looks like the complex thing." I guess it also lets you discard the geometry of the object (assuming you don't let the camera get close enough that you have to switch to a fully rendered view) which I can see saving a ton of memory for very detailed objects.

I remember reading a bunch of stuff a decade or so ago about image-based rendering using techniques like this for super-high-detail large scale scenes, is that all run-of-the-mill graphics engine stuff now or did we get enough horsepower to just keep throwing polygons at it?