Ok, well then I'd like Bethesda to release the specs for the Net Immerse File format so that we can write our own plugin! We don't know what half the options of NIF do, we're just guessing. And when it comes to writing a plugin for 3DS Max, Maya or Blender, you need more than just guesses.
To answer Benrahir's question: yes, the NifSkope team is RE'ing the NIF. They've figured out what the various elements do by changing them and seeing what effect that has on the model. For many of the unknowns, they haven't been able to figure out what changing them does. SaidenStorm is the guy to talk to about that.
Unfortunately development of the NIF exporter for 3DS Max and Maya stopped a long time ago. And I don't wish to use Blender.
We have more than enough spec covered for nifs to have working exporters, the Blender scripts work just fine. (Max/Maya work fine, they're just outdated) You don't need to use Blender to do your work, there's no reason you can't model in another app, and use Blender for the final export.
Many of the unknowns that do nothing are either not used by a particular game, or actually do nothing, making it hard to pinpoint things
I believe the majority of the missing sections are in Bethesda's implementation of Havok, which they certainly won't release a specification for.