2.7 Protocol Expression and Transport Independence

"To make semantic data interchange as simple and extensible as possible, another XDI design goal is to define the XDI semantic data interchange protocol in XDI itself. This means all XDI messages must be valid XDI graphs, and all XDI data sharing operations are XDI graph merge operations."

Comment: while this may indeed make interchange extensible, I think that defining messages with XDI graphs is more complicated and harder to understand. There are other ways than XDI graph based messages to achieve transport independence.

Question: what is the motivation for XDI graph based messages?