Maybe i experienced, if you didnt wrote about this, then sorry :)
If a target isnt heading on you, only around, or beam, or heading home, but use ECM, you will get a stable target in the ECM, if he is in the burn out range, but if you lock him, that is like you lock an ECM. The only advantage is if you broke lock, you know the range, and if the target is already should be in engage, you can shoot your AMRAAMS. I never fly in F-15, but i think its true for all aircrafts :)