o.k. maybe not most often used, but it really is one of the features i need photoshop for. i did not find any other program that would do this thing so easily and effortlessly.
and no, the proposed plug-in does not cut it. actually you have to move the *opposite* corners of the selection in the plug-in to compensate for the perspective. and not on the real image but on a small thumbnail of the image in the plug-in window. not really intuitive and easy to use at all.
btw, i use it for cropping pictures of slides taken with a camera during presentations, lectures, etc..