Earlier quoted context omitted.
Laugh if you will, but I've found sketching out mockups in PowerPoint to work surprisingly well and about as fast (or faster) than paper since I don't have to redraw the whole thing after too many changes are made. And to your point: You can’t click a Photoshop mockup. This is probably the number one reason we skip static mockups. They aren’t real. Paper isn’t real either, but paper doesn’t have that expectation. A P…
That's a clever idea, and if it works for you, great. In fact I think it's a much better use than PowerPoint's intended purpose of crushing your soul bullet point by bullet point: http://norvig.com/Gettysburg/sld001.htm It looks like you can sort of do this in Google Presentation too, though you have to figure out the URL to link to other slides.
Most of the actual graphics I use in the mockup I've made in Photoshop anyways, and just put them in the mockup, so those are reusable art assets on the final page.
(plus you can even mockup smaller gui details like drop-down menus if you are clever enough, though it's usually more work than it's worth)
I've found that most of the MS-Office tools can be abused to do a pretty good job in roles far removed from their original intent.