Actually, there was a fiendish dwarf creature in 3E, whose name I can't remember, that got "merged" with the duergar concept for 4E. That's where all the fiendishness and beard quills came from.
Not everybody appreciated the change to a classic D&D creature, but I dig it.
The whole beard quills thing is odd, but I assumed it was because they were creating a greater connection to "bearded devils", which have been around for quite some time and all that.
Ie, the whole duergar <-> durzagon <-> barbazu circle of "life".
Can I be so bold as to ask you to explain how/why?