Observed abuses aside .... and anything can be abused .... the code does provide for specialty luminaires to be installed closer to stored materials than you might expect from the excerpt posted above.
For example, there exist illuminated closet rods that, by their design, can't help but place the cloths much closer to the light than 12".
I expect hat with the expanding use of CFL and LED light sources, as well as the legislative assault on the old-fashioned incandescent light bulb, that closet lighting will soon stop being such a 'hot' topic.
Now, does putting an enclosed bulb, or a CFL bulb, into an ordinary fixture make that fixture either enclosed, or fluorescent, under the NEC? Good question.
Early on, the IAEI took the position that no, it does not. In a like manner, California took the position that such did not make the fixture energy efficient either. Both justified their positions in that you "might" later put an incandescent bulb in the fixture.
I do not see anything in the NEC, or the various energy codes, that allows for such an interpretation. Not only is such an approach an affront to our concepts of right and wrong (do we ban cars because they MIGHT be used to commit crimes?), but Article 90 of the NEC makes it pretty clear that what 'might' happen in the future is beyond the scope of the code.