Does anyone know exactly what purpose that foam backing for the vent seal on the Kirby Classic and Classic Omega is supposed to serve, for example, like quieting the motor down?
Mostly to divert the flow of air coming off the cooling fan. The air from the cooling fan was directed at the floor. Thus stirring up any dust, animal hair and such. Buy placing the foam pad over the vent helped to dampen the air flow yet allowing the air to exit with little resistance. Therefor reducing disturbing of debris surrounding the machine before they could be vacuumed up.
You could if you want to. For the most part those have been done away with as it is just one more thing to get into the proper position when reassembling the motor shell. It was also there to trap the carbon dust from the motor brushes.
I for one really don't worry about the carbon dust. Comes down to a matter of personal preference as it doesn't effect the performance of the machine. If you do make sure it is an open cell foam or the motor will over heat. Closed cell foam will not let air flow thru.