There _are_ qualifiers to 310.15(B)(2), in the form of the exceptions that permit certain conductors, which are clearly expected to carry current, but which are not _counted_ as current carrying conductors. Eg. the permission to not count a neutral which only carries the imbalance current in a multiwire circuit, with the qualifiers that insure this is only used when the neutral actually doesn't carry current.
The example set of 12ga wires (which under current code _must_ derated to 15A) is different from a 14ga switch loop in a very important fashion. If any _one_ of these conductors is actually carrying 20A, then the other conductors cannot carry much current or the OCPD will trip. In other words, if any one of these conductors is actually carrying the full circuit current, the other conductors are _not_ carrying current.
If I stuff a conduit with a single 'hot', a single 'neutral', and 11 unused conductors, then under current code none of the conductors would be derated.
-Jon
