All it usually needs is one word 'reds' to prompt the ATCO (or at least me) to drop the bar. Not too much in the grand scheme of things.
I'd far rather get 45 a/c away in an hour and have a few query me when I forget to drop the reds than get 47 away but have them all crossing over red bars because they read back their line up clearance correctly.
Hell, I'd rather get 20 a/c away in an hour than have that happen.
That an a/c has read back his line up clearance correctly is no guarantee that it will line up correctly. In fact, we've had quite a few incursions during the day by aircraft who didn't have any line up clearance. In those cases, the red bar is the only thing stopping it happening at night.