I found this an interesting point. My instinct is that generally pages ought to be easy to read but there can be exceptions. I think that the introductory pages should be easily readable but that they can link to more complex pages depending on context. For example, my daughter is currently designing a website for a module of her Masters in Geoscience. The topic of her website is Magma chamber dynamics and chromite formation. The module outcome is to produce an accessible website with an illustrated literature review. This is a highly complex area covering advanced physics and chemistry and I can see no way of simplifying the reading level once the reader is past the introductory page but surely this is fine. If the reader can read the first page easily and realise that it is not the material they need then I think that this is what is required.
As part of H800, I looked at the OU site introducing the Evolution Megalab project involving citizen science. It is designed for the general public so the majority of the pages are written at a simple level but there are also people who have a more specialised interest in science and so the factsheets are highly scientific and at a higher reading level.
I do feel that guidelines for reading level should be included as part of accessibility guidelines. It is important to make general web pages accessible for everyone. My daughter is deaf and has many friends whose first language is sign language and they often struggle with higher levels of written and spoken English.