This would be incredibly helpful for me. That way I could look at the synopsis and remember what passage I’m talking about and then the Document Notes to get an idea of what I want to do with it.
Considering that a lot of people keep a lot of text in the document notes pane (er, I do for one), this could quickly get insane trying to view it in the outliner. What you can do though is keep the inspector open while viewing outliner in the inspector, then just click on the row for the document that you’d like to skim the notes on. Given that you can really only read one spot of text at a time, this isn’t really a loss. You can see all the synopses together in the outliner and then just click or scroll through with the arrow keys to get more information in the inspector.
That’s my two cents, anyway. I don’t design the program though, I just use it. 
This would not be possible because the notes are rich text and the outliner is only capable of plain text editing, and nor would it be a good design choice given the amount of text that might be stored there, as MimeticMouton says.
Thanks,
Keith