[boost] [iterators] iterator_facade

I am looking at implementing some iterators based on the design paradigms outlined in http://www.boost.org/libs/iterator/doc/iterator_facade.html In the first instance I am not able to use templates at all, but I would like to implement it in a template-friendly fashion, and hopefully make it fully templated in due course. My question is why is it recommended to implement iterator_core_accessfunctions as private static members of a separtate class rather making them a base class of iterator_facade which could be privately inherited and then remove the need to make iterator_core_access a friend of Derived?