BAA is fined £2,000 for every bag that does not make its flight by the airline, so it is in their best interest to get it on the flight on time, that includes transfer baggage.
Once it is on the flight it seems that then it is not so important, seems most bags are lost in arrivals.
I am amazed each day I walk through T1 arrivals at the sheer number of bags left lying around at the end of each day unclaimed for whatever reason.