I think a major advantages for me are easy bookmark-ability and sharing via URLs to people who may not have the app. You can bodge workarounds for those things

I think a major advantages for me are easy bookmark-ability and sharing via URLs to people who may not have the app. You can bodge workarounds for those things in an offline app, but now the bookmarks aren't in my browser or I have to copy them to it, or the people I share the doc to are just looking at a browser rendered viewer or something. While Docs hits 95% of my needs there's still that 5% and I suspect most of those are held back by the current implementation architecture. Hopefully moving to a Canvas based system will enable them to more easily add complex features.