I note that in
http://callfortesting.org/gnome3/ that the Gnome Shell requires it. Per
https://forums.freebsd.org/viewtopic...31797&p=191770 it appears that the initial implementation of Gnome 3 on OpenBSD (and only on OpenBSD) could use Fallback mode, and this was corroborated in the CVS logs for gnome-session on OpenBSD, where patch 1.219 for Gnome 3.6.1 mentions it as going away in the next release:
http://www.openbsd.org/cgi-bin/cvswe...ssion/Makefile