You raise very valid points!
Unfortunately the flexibility is not something I leverage that often I always prefer to keep installs simple wherever possible. What I do however come across a lot is customer environments with statements like 'what do you mean design documents' and 'X used to look after it but we don't talk to X anymore'. Don't get me wrong I charge by the hour but I always feel guilty/sorry for the customer when I have to turn around to the customer and say 'brace yourself, this might be expensive'.
I agree completely with the document, I always ensure customers have an up to date low level design document that outlines anything that has been configured on the appliance.
Installation and Upgrade guide is a very good idea that just hadn't occurred to me; thanks.