Estes Big Daddy - 29mm Mod

With LP motors, the MMT tube holds the engine block at its forward end, and I suspect the feeling that it's necessary came from always having one for this reason. If a thrust ring is used, you really don't need a MMT tube at all. Fin tabs, if any, can be notched to interlock with notches in the centering rings, and the rings' ID reduced to the engine diameter. That makes lining the engine up with the forward ring a little bit of a pain, so the tube is handy as a guide. The tube also aids during construction, to get the rings parallel and their notches (if you've got them) aligned on the MMT subassembly. And it can be handy for other things, like the baffle I built into my L1 Big Daddy. But it's really not necessary.

Hmmm, I had surmised that having an MMT the full length of the motor casing would protect recovery hardware from very hot motors. Is this incorrect?

Only if you've got recovery hardware positioned that low in the airframe. Normally, the recovery hardware is above the motor anyway, and separated from it by the forward centering ring, isn't it?

Even if the motor sticks very far forward of the ring, the propellant is burning from the middle of the grain out (not always on center, but still...) so combustion is up against the liner for only a short time. And that's the liner, not the case. I don't know how hot the case gets, but I would think that reasonably robust materials could take it. After all, the mount tube takes it.

motor cases usually stay below 400F(when they get hotter the rocket is generally destroyed)
Rex