Centaurus,
In response to your question:1/ Its my company standard ops, 2/ picture a scenario where you are on descent into an aerodrome with reasonable terrain(and its associated peaks and troughs) as you fly over a peak, rad alt goes off, (if it is within 2500ft), then as you fly over the trough, rad alt is lost. then comes the next peak, rad alt goes off again and so forth. By setting rad alt DA to 1000ft it only gives you notice when you need it. Remember enroute LSALT, MSA's etc give 1000ft terrain clearance.