My goal is to always be able to show to some kind of source, for anything that I type into my tree. Adding documents and links is a great feature, but a lot of information is best referred to by providing the book title, ISBN, and other meta-data unique to the book. Also providing which pages within mentioned book the interesting information resides in would be of value.
So far, the closest thing is to create my own custom document, where I simply write the same things.