The shape-note system is by design a pedagogical tool.

The genius of the system is that it combines a relative interval based system (like the do-re-mi of solfeggio) with the conventional staff-based system that can be read by conventionally-trained musicians. And it combines the two in a simple visual representation.

A relative interval system is much more informative to beginning singers, who have no fixed-pitch instrument to associate the staff notes with. And it corresponds with the way we hear and produce sounds better than a fixed-pitch staff plus key signatures.

In teaching people to sight-sing, I have found that some effort spent to impart understanding of the scales and what the various intervals sound like in context bears great fruit. There are many ways to teach this, and this is exactly what the traditional "do-re-mi" is for. But in that case there is still considerable mental calculation involved (at first) to associate "the-third-line-from-the-bottom-in-treble-clef-and-one-sharp" with the sound of "mi." In shape-note singing, the shape gives you an immediate indication (alas, though, in fa-so-la rather than do-re-mi -- we can't have everything I guess).

I haven't read the study cited. I wonder if the same effect would have been gained by any relative-pitch teaching system, or if shape-notes in particular are more successful than other systems.

As to whether the shape-note system is of help to instrument learners, I doubt it. Producing pitches on most instruments is a matter of physically manipulating the controls of the thing in a certain way, whereas vocal production is primarily aural.
Not that there is no connection between aural learning and instrument playing, but the primary focus of sight-reading on an instrument is mechanical.

Having said all that, I would add that the music associated with shape-note singing in the Southern US is an aural experience all its own. Love it!

-Glenn