I suspect the worry is that if patrons can't get ADR's due to a restaurant's being booked up, then they won't show up at all because they'll assume if they show up they'll just be turned away -- much as happens at other WDW restaurants.
Then, if those with ADR's also don't show up, the restaurant is empty. I've seen this outcome more than once at WDW restaurants -- no ADR's available and a 1/2 empty restaurant. I suspect Planet Hollywood is trying to avoid this outcome. And, what's the best way to avoid this Catch-22, don't offer ADR's -- make everyone wait!
Of course, this risk, then, is that the wait is too long and people give up and leave before they are ever seated. (That would be me -- life is too short!)