That's an interesting idea however I assume I'd have no access to media when I wasn't connected to the NAS.
When I used to use Windows Media Player the library was made up from a random collection of folders across local PC / NAS (and other PCs if I so wished). If I added a new album to the NAS storage I'm sure it would appear in the library(am I imagining that?). If I added it to local storage again it would appear in the list but I could carry it around with me.
I want some music I can carry around on the Air but books, that I don't want to read on the Air but do want to sync to the iPad, I want on the NAS. However I don't want to manually have to update iTunes each time I add a new book. Am I asking too much?