The requirement was copied from the FAA where the BFR can be replaced with a Check Flight on another aircraft.
The JAA reasoning was that pilots sholud be periodically subject to a controlled environment, those who fly for a living operate in such an environment. The process was to ensure that PPL holders who could go unchecked ad infinitum should be checked every other year. So far it has not generated any issues in the 17 years it has been in place.