The Quiesent callback is not absolutely required. SyncEvolution should keep the view open even if that call returns "org.freedesktop.DBus.Error.UnknownMethod".
Currently a ViewAgent must implement the callback.
Patch pending in branch, will be in master soon:
Author: Patrick Ohly <email@example.com>
Date: Fri Nov 30 14:42:22 2012 +0100
PIM: ViewAgent.Quiescent() optional (FDO #56428)
Document the new callback, make it optional and add tests, also for
the old behavior of "close view when ViewAgent methods return error".