To stop RMH fire brick cracking

I have heated my home with a traditional RMH for over a decade, and have had to perform small repairs/maintenance on my feed tube every year. Usually I just beef up the clay mortar and cob ring at the top of my fire brick feed tube, as it holds everything secure, and it tends to get brittle and crumble over the coarse of a season.  
But the past 2 years, I haven't had to fix anything as I finally realized I could just not clean out all the ashes each morning. Leaving a couple cups of ash seemed to provide the magic heat buffer, and the cob portions are as beefy as when they were put in. I'm surprised what a difference the ash residue makes, so I thought I'd share.

I've heard Ernie Wisner recommend this in a video or two.

Leaving some ash on the floor would certainly buffer and protect the bottom of the feed tube and burn tunnel, but I don't see how it would affect the top of the feed tube. I would be interested in finding a mechanism that would explain your experience. I only clean the ash from my J-tube every week or two and find it helpful, though I can't speak for the top as I have a steel insert that routes secondary air to the P-channel.