It is much easier to use the tools provided by dbus to
  do the session startup. Debian/Ubuntu does that too, so
  I suspect that the internal startup code is not well
  maintained.
  That's why I've changed the gnome session startup in the
  default gdm config file when I did the update to 2.18,
  see pkgsrc/x11/gdm/patches/patch-aa.

That makes sense, but it's unfortunate that just gnome-session is
trouble; that's what many people have in .xsession