It has been decided by the CLE Team on 4/12/2012 to deprecate the CourierService and its implementations.
Some of the reasons include:
1) Presence is no longer using the courier service as of 2.9 see
2) The BasicCourierService does not manage its own data structures efficiently relying on its consumers to do the "right things" which makes it prone to errors.
3) There have been leaks already reported see
4) The only core tool still using it is chat 2