The F-16 EME Testbed can be used in place of an operational aircraft in many applications. This resulted in a significant savings in cost. The aircraft used are empty airframes. Most systems that are evaluated at the facility do not require electrical or data connections to an operational aircraft. For these measurements, empty airframes are best.
Some systems require interaction with the aircraft.
Use of operational aircraft for these measurements would require extensive, expensive modifications to the operational aircraft due to safety of flight requirements. The experimental nature of some tests would also require use of the aircraft for extended periods of time.