... and nobody sells the threaded bit that slips on over the MMT, only comes on Estes plastic jobbies.
No....because it's molded into the aft ends of the BT-55 and BT-60-sized ARFs, plus the fin cans that are part of the Mix-N-Match 55 and 60 sets plus, I expect, some of the recent E2X kits that have screw-on motor retaining rings (though that would have to be verified). It is not a separate part.
Was out with my eldest son and his son on Easter afternoon flying the Flying Colors/Booster 60 combo and having a great time. I have also used the boosters with models made from the Mix-N-Match sets.
Perhaps the best use for the boosters, as sold on clearance at HL for example, would be to use as ready-made 24x70 mm motor mounts/fin cans for BT-55 or BT-60-based scratch builds....just peel off the upper paper portion and replace with the body tube of your new design. [see examples in next two posts] Since all the fins used on the ARFs and the Mix-N-Match sets have the same interface, you can swap the trapezoidal fins that come with the booster for another shape if you want.
The reason they come with an extra stager ring and why they are available separately is that if you fly them very much that ring gets cooked pretty well in the process of staging and eventually they'll need replacing. Not that I've had to do that yet.
samb's thought above about just using a JT-55 or JT-60 in place of the stager ring and using the booster to gap-stage to an existing or new model also makes sense. Just be sure to vent the booster unless you use D12-0s exclusively in it. On the ARF/booster combos they will separate before the sustainer lights sometimes if you use a C11 instead of a D12 in the booster. (This is also a good idea for the similar booster for the PSII Majestic/Ascender if using an E16-0 instead of an F15-0 in it.)