Jeepers...
Bookworm - do you really lookout ALL of the time? Of course not - it is not possible.
Yes, PCAS only alerts you to cooperative threats but having an alert against, say, half the threats is a 50% greater chance that you will not hit someone. I have already said that PCAS is not an infallible aid - it is about stacking the odds in one's favour.
And I certainly did not say that you ONLY lookout when PCAS alerts - in fact I specifically highlighted the importance of a good, disciplined scan - but the PCAS gives you an added cue to STOP any in-cockpit task and lookout like a demon.
Too many of the responses regarding PCAS resemble the 'Ostrich' approach to threats - if I can't see it then it can't hurt me!