Ah, just found a nice paragraph I'd never noticed- 366.23(A) allows us to ignore derating when there are less than 30 conductors in a sheet metal auxillery gutter. No exception for plastic, though. Very handy exception that could be easily extended to apply internal cabling inside most cabinets, I'd think.
310.15(B)(2) allows you to ignore derating factors on cables < 24" not in raceways, but gives no such exception for cables in raceways.
Copper cables act as their own heat sinks, transferring heat within the cable. Annex B.310.15(B)(3) goes more in depth into this, allowing you to use the greater ampacity if the higher derating factor is no greater than 10% of the run (max 10'). Which is VERY handy! Unfortunately, Annex B only applies to engineers, though, sorry! At any rate, this isn't additive with other exceptions, but the engineering rationale that the 24" bundle/nipple, cable gutter, etc, exeptions are based on.