cd318: Yes, as far as I can see, Linkwitz did recommend this, including in his own designs. I use it sometimes but I agree with SL (as quoted by REG) that it must be adjustable. I think that a shelf down 1-3 kHz is a bit broad.
As you say, other reasons for using this dip have been stated, most prominently necessity due to manufacturing limitations.
I am no theoretician, but I believe the need varies according to the design of the speakers (in particular their directivity with frequency), the type of recording, and so on.
Where there seems to be agreement is that around 3kHz the ear is most sensitive. Playback that is hyped there will be irritating to many people, and that can be relieved by adding this dip.
Quality audio is complicated IMO because there are no standards for recording; mics are colored; speakers are colored; and no two home setups are the same. Of course, there’s also the issue of how one quantifies "quality." Not trivial!