OK, here is my recollection of the history of this prohibition. There are two fundamental problems with lampholders such as this installed in closets. The first is that they are rated typically for up to 600 W or so with NO warnings as to use a certain wattage bulb. Now it is unlikely that the "average person" will put much more than a 100-200 W in there it has been established that 100 W and higher MAY develop bulb surface temperatures that can ignite light combustibles. Here you have essentially an unprotected bulb. Secondly, it is not usually predictable how the "average person" will configure (reconfigure) a closet for storage, or literally cram the space full. 100 W and some smaller wattages can easily develop bulb surface temperatures that can ignite light combustibles WHEN INSULATED, as by a comforter or other storage that falls forward from a shelf or is pushed by other storage against the bulb. (Joe, your photo is a good example of how the proximity of storage could allow this to happen!)
There were enough documented cases of fires originating from these installations to convince the NFPA committee to restrict lampholder use. The change occurred some time ago and there may be additional justification in some of the older NEC Handbooks.
