Once upon a time, information architects were handed a load of content and functionality, and it was their job to sort it all into a cohesive, cogent model. As would often happen, there were some items which just didn't fit into the mold, and so these extra pieces were put in a clearinghouse. Two examples would be the 'FAQ' on web sites and the menu bar on the PalmOS.

Logically, if you have an informational site, people are coming to your site with questions. By that token, their questions should be answered by the site, not by an additional FAQ. Its existance comprimises the intention of the site, unless you choose to put all your information into the FAQ.

This thought isn't fully fleshed out yet. I might add to it later.