The Avid ones I use for my BB7 brakes seem to work just fine. Unlike wafter's mine are under a little bit of tension all the time, which seems to be enough to hold one part still while the other rotates.
In fact when wound "together", reducing cable outer effective length, so reducing tension in the inner, one part can disappear inside the other. Even when extended there's barely enough "inner" to hold on to.
Maybe it's necessary by design? If the "inner" section had some sort of knurling, would that not mean that the inner diameter would have to be reduced, unless the body was made extremely thin? Perhaps the design restrictions are to do with the diameters of cables, inner and outer.