without their shields that would become alot of invisible power
if, for instance, the damaging effect said 'lasts for 10 seconds' instead of as long as the shield persists, then if it's broken there wouldn't be an obvious indicator that Naut is currently throwing out more damage
If the shield is up, its a visual cue that he's going to hit harder, and when it breaks you know that he's lost some of his power and it's a little safer to go in, but without the persisting limitation you couldn't be sure how long his damaging buff is going to last, especially in the heat of a fight
the damaging effect of his shield is to compensate Nautilus for building tank, as his shield scales of of maximum hp. The more health he has, the longer his shield will last, maximizing the damage