(By the way, it did not that a CO reduces the size of the Airy disk slightly, the energy transferred into the rings has to come from somewhere. The disk is dimmer, therefore slightly smaller, that energy is transferred into the rings which smears the image thus reducing fine scale contrast. Of course that's a small effect... )
Jon, seeing is an interesting thing. You and Dave hit on it pretty much. All I would add is "tuning" a scope for prevelent seeing conditions might be a consideration. I conastantly rave about seeing in the tropics. Seeing a nearly perfect diffraction pattern almost constantly is "pleasing." It seems well suited for a 6" aperture, probably more, almost every night. I still have to wait for those almost perfectly still moments, but they are not so few and far between. So, calm seeing affords some jaw dropping views of lunar and planetary frequently throughout the observing session. It's both pleasing and working at full resolution and contrast a 6" is capable of.
As you alluded to discussing larger apertures shining in those calm moments, that's a scope can operating near it's theroetical, unaberrated MTF. When the seeing calms the theroetical plot settles toward "testbook," well, those are the jaw dropping moments in any aperture.
To your above, yes, the CO dims the spurious disc to some extend relative to the surface area of the obstruction, I believe (1 - co^2) normalized. However, the obstruction also adds diffraction to the system and changes the actual pattern of interference forming the Airy disc. The Airy disc is made smaller and the rings shift slightly due to added diffraction as well as obscuration. Both are small effects [in sum according to Vlad, et al, (1-co^2)^2]. When the seeing is nearly perfect those changes can be detected at smaller exit pupils and especially on equal tight doubles. And probably on lunar and planets, too, IME.
It usually bugs me a tiny bit some obstructed designs, CATs especially, and Newts are usually associated with seeing conditions, thermal issues, and collimation. These conditions, which can be minimized and optimized and are not necessarily intrinsic induced aberrations of these designs, are often toughted as reasons why CATs and Newts are Goliath while the humble refractor is David, who slew Goliath...in bad seeing. Its not a story of some noble short guy beating up the evil giant, rather its a story of an inferiority complex. A rationalization of owning 4" of aperture offering pleasing views frequently. I experience the same "pleasing" experience in my own 6" CAT because it is well tuned and operating in excellent seeing (=/> 8/10)almost nightly.
Refractors are great scopes, no doubt some of them high end, that give "pleasing" views. And yes, it is nice to use both designs where they excel. Not trouncing refractors here, just trying to put all those bad things that make Goliath a bad guy into some perspective. There is at least one observing condition I can think of where a 6" unobstructed, finely hand crafted scope can trounce a descent 12" obstructed scope...when the latter is at the bottom of the ocean. The seeing sucks down there.