That's why knockdown give 2 or more extra hits. One when he's on the ground (maybe 2 if you're using one of the shorter maces), one with a kick-slash when he tries to get up, and who knows if you might knock him down again after the kickslash and continue the cycle. Knockdown > Stagger
Alright, but I'm just considering polearms here, and most of the knockdown polearms are not very good weapons in comparison to other polearms. If you're talking 1handers or 2handers (shorter maces?), obviously they can't even have stagger so that comparison is purely academic.
It's also possible to get chain staggers, which is far more likely to happen than repeat knockdowns (which afaik can only happen if you do the kick maneuver, since you can't knock a downed person down until they get back up). And like I said, you don't have to adjust your positioning or your aim whatsoever upon a stagger, it's easier to hit them, but if you knock someone down you have to be more on-the-ball.
I have managed to kick someone after a stagger-hit before, as well.