moti – good one to bring up! I too, am annoyed when a Shatz does that, or when someone is saying kaddish and ends off “D’amiran b’alma v’imru” and then waits to say “amen” together with those responding. Please think about what you are saying! The one who is saying kaddish is telling the people to say “amen”, so tell them that! V’imru amen – and say amen. V’imru – and say. And say what?
This problem is also aggravated by the fact that sometimes the “nusach” (i.e. the tune that the chazzan uses) lends itself to making this mistake.