I believe it will be useful. We all know that things like that generally don't get read unless people are pointed specifically at it. A select few will read the top-level FAQtop-level FAQ and come to wrong conclusions about topicality.
In my experience of correcting the wrong-headed, citing the exact bullet-point they failed to read (or comprehend) ends up earning an oh, sorry
response more often than outright belligerence. In the cases where such a pointer earns a, How does THAT make sense? response, having a more detailed description available of why we do it that way will help.
Additionally, this would put such a document within the edit-powers of both the mods and our users. The FAQs of the original Triad sites are the only FAQs where the mods do not have edit privs, that lays firmly in the SE staff. This would allow us to better keep the long-form FAQ up to date with community standards.