<Similar to waiting for "v'imru amen"s to end before responding "amen" (particularly before starting Musaf, where many tend to sing the "amen" together with the chazzan.> Why wait? "V'im hashatz ma'arich b'nigun shel 'v'imru amein' yomru kakahal 'amein' miyad," says the Magen Avraham in 124:14. EMT