As I said I can see both sides and my feeling is the NEC is on the same page as our local inspectors.
Consider the following exception to the derating rules.
310.15(B)(2)(a) Exception No. 5: Adjustment factors shall not apply to Type AC cable or to Type MC cable without an overall outer jacket under the following conditions:
(a) Each cable has not more than three current-carrying conductors.
(b) The conductors are 12 AWG copper.
(c) Not more than 20 current-carrying conductors are bundled, stacked, or supported on “bridle rings.”
A 60 percent adjustment factor shall be applied where the current-carrying conductors in these cables that are stacked or bundled longer than 600 mm (24 in.) without maintaining spacing exceeds 20.
To me this exception demonstrates that the NEC considers MC supported on bridle rings every 5' to be bundled.
If they call this bundling I don't see how we can not call NM through studs bundling.
MC hung on bridle rings always 'fans' out some.
Really what I believe and I know of someone putting in a proposal along these lines is this whole issue should go away for NM in dwelling units as you will never have all the conductors fully loaded for any length of time.
Here in MA we still use a lot of NM in commercial jobs and I have found some pretty warm bundles of NM in these locations.