Most often they appear on headless instruments (Steinbergers, Hohners, etc.) as you already having something keeping the ends of the strings in place, another fret is easy to do.
The Dingwall Voodoo I owned had one, but it also had string guides like that Hofner. It would seem that if you were going to mount a string guide to keep the strings in place, why not just use a nut? If the guides are wide enough then you have the option of using different gauge strings, other than that I think that price is the determining factor (nut is easier and thus cheaper, etc.)
My luthier didn't like the brass nut on my bass when I brought it to him to file it down a bit because it eats his files, but I like it now that he's done with it.
Gaddis Tippett Custom 5, Circle K Strings, Micro rig: Ibanez IBZ10A acoustic guitar combo w/chorus!
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