PowerPoint prototypes are a great way to show someone how the flow of an interaction might work and even better, you can send them the .ppt file to view before or after your demo, not to mention ensuring that almost everyone you work with could (if you want them to) contribute to the PowerPoint deck since the application is nearly ubiquitous.
As an aside, I’m always a bit impressed with the ingenuity of people who live in one application for everything, and PP certainly can let you do that. I’ve known many people that use PP for note taking, article reviewing (guilty!) and of course outlining (it’s better than Microsoft Word). However, this is nothing compared to the people who used to live in Lotus 123 including writing memos and even formatting floppies. (Ah, floppy disks.)