I can certainly see the benefit in mentioning type of aircraft especially at uncontrolled fields as there will be no help from ATC. When you have a mix of MEP and SEP in the circuit it is very important to know who/what is ahead or behind of you with the varying speeds on downwind. Just think of a Cessna at 90kts on downwind with a seneca doing 120kts behind him! Type of aircraft very important then to allow adequate spacing.