Vent motors like blower motors are designed to work against a restriction. The restriction disc is just for that purpose when the total venting configuration is lacking the necessary restriction by its design. It's a difficult concept but by using an Amp meter you can see that the blower motor actually works harder when there isn't the restriction of the ducting. Very nice video especially the part detailing that safety devices are very likely not failing but reacting to an unsafe condition. By design they are generally very reliable and in themselves very functional.
