I experimented with this and found it powerful. It pushes you to clarify your ideas. For a product idea I made, which I thought was clear in my head, still proved challenging to write as a press release. It took me a few hours. Things became much clearer after I wrote it.
At the same time, it was almost fun to write the FAQ. I addressed the questions some of the users had asked, and again, this gave me new ideas, and new questions to think about.
I did this exercise at the end of the design, to communicate the benefits better, but I have a feeling that it would have been instrumental in the earlier stages.