Bottom line up front. I think that backing plates might serve an unexpected purpose. They keep rubber worms from blocking rotor cooling vents.
Details. Last year I went thru a brake cooling obsession phase and ended up fabbing my own solution that put air flow to the rotor inlet just where I wanted it. My solution doesn’t attempt to seal off the rotor inlet, it just directs air at it.
Last night I was changing a cracked rotor. I noticed that there were a couple rubber worms partially blocking some of the rotor’s cooling vents. As I recall, everytime I’ve taken off a rotor, there’s been a worm or two stuck in it. Blocked off rotor vents have to play hell with homogenous cooling.
I bet that backing plates that cover most of the rotor inlet extend rotor life by blocking out most of the rubber worms that fly towards the rotor inlet. Although the rotor faces certainly get hot as heck, the rotor inlets wouldn’t get nearly as hot. I’ve no guess how long the average rubber worm sticks in there until it’s softened enough to get ejected, but they’re certaily getting stuck in there and blocking air flow.