When I'm writing a paper, I usually prepare a folder in Paperpile in order to gather relevant literature. One folder per paper. I'd guess this is quite a common approach.
I would find it very useful if the Google Docs sidebar add-on could show me the list of citations from a specified folder from Paperpile. I would also like it to show me which of these citations are already included in the current document. This way I can easily see which papers I still need to review. I would imagine it being implemented as an additional [ITEMS FROM...] drop-down.