As of dbus/1.10.6-1ubuntu2 (the relevant change seems to have been in 1.8.12-1ubuntu1), the Upstart job creates that directory but then doesn't do anything with it, because the session bus address is now stored in XDG_RUNTIME_DIR.
This is one of several ways in which the Upstart job is far more complicated than it needs to be, for which I'll open a separate bug.
As of dbus/1. 10.6-1ubuntu2 (the relevant change seems to have been in 1.8.12-1ubuntu1), the Upstart job creates that directory but then doesn't do anything with it, because the session bus address is now stored in XDG_RUNTIME_DIR.
This is one of several ways in which the Upstart job is far more complicated than it needs to be, for which I'll open a separate bug.