OK, let me preface this by saying that I have not owned, played with, seen IRL, studied on the web, or read the manual for the Frontier. So I could be totally off-the-wall with this.
The Frontier allows for 20% to hang off the front edge of the vehicle seat, right? And the recline block is completely in that front 20%? So there are some install situations where the recline block doesn't do diddly to recline the seat, nor to keep the vehicle seat from compressing the harness.
Also, why would it be required to be not reclined in booster mode? European boosters have the seat of the boosters slightly reclined, because it helps with submarining, belt positioning, and abdominal forces.