When training on the 320 my instructor told me this:
He thought if the flight path is all programmed in, and the aircraft will follow it until the intercept of the ILS, why not press the APP button early? - It is very dangerous, because the ILS reciever then controls the aircraft movement (speaking figuratively) if it recieves anything. So pressing it over the Atlantic would probably do nothing, but pressing it anywhere where you might recieve a signal of the same frequency as the ILS you intended to intercept will cause a capture. That might have some negative side effects cruising at FL350, if the machine pitches down and starts turning...
What you are doing is escentially giving radio signals a higher priority than the FMS signals towards the autopilot.
Boys, it is not a toy!
Nic