It should be incumbent on the joining pilot to visually identify all circuit traffic before joining the circuit.
Also, at most small airfields, it is likely that the person operating the radio will at the same time be taking landing fees, making the sandwiches in the cafe, answering the telephone, and generally handling any number of other daily flying school/airfield tasks, which means they will often only have a sketchy idea, at best, of what is going on in the circuit.
Just to be pedantic, the R/T Licence gives authority to operate the radio from an aircraft, nowhere else. Don't do it if the CAA are about.