I can kind of understand using this as a layout rough, but as a proof - how can it be? A proof is just that, with the right words and the graphics/images. How can you possibly sign off or agree to something that does not contain the right material to start with? I just think its kind of extra work, when you could just work with the real stuff to start with

Another cool effect for stamps in PDFs is to make the stamp somewhat transparent by having it overprint in Illustrator or InDesign. The stamp can then be laid over an image and you can still see some of the image come through.