[pas]

... well, it makes sense to be able to do a "join" with the `users` and `documents` collections, use the full expressive range of an aggregation pipeline (and it's easy to add additional indices to MongoDB collections, and have transactions, and even add replication - not easy with a generic filesystem)

put all kinds of versioned metadata on docs without coming up with strange encodings, and even though POSIX (and NodeJS) offers a lot of FS related features it probably makes sense to keep things reeeeally simple

and it's easy to hack on this even on Windows