I recently started using layers in InDesign and have been blown away by how useful they are.
I could see layers being equally powerful in Dorico. Place bowings, analysis, chord symbols, all sorts of other elements on a second layer. Then a toggle displays or hides selected layers with one click. Also, printing gives the option of showing or omitting those elements on export or print, without the need for a duplicate layout.
The essential difference is that there’s no ‘collision avoidance’ in InDesign. Adding content has no effect on the position of other content. (The only exception I can think of is text running around a graphic; can’t remember how different layers would affect this.)
And indeed, you might very well want text and graphics overlapping with some layer effects.
Adding bowings, chords, etc, would affect the spacing of the notes. So either you kept the spacing for objects in hidden layers, (which might well look odd); or you’d effectively have a new layout.
I’d tentatively suggest that it would be non-trivial to implement; and I’m wondering what the use cases might be, that wouldn’t be served just as well by methods for showing/hiding stuff on a per-layout basis.
I remember going to a showcase for InDesign in about 2000, and seeing different texts in different languages on different layers. I worked for an illustrated book publisher, and we combined print runs in different languages, switching the black text plate mid-run. This was a game-changer compared to Quark’s ludicrous approach.
However, on a page of music, you couldn’t swap the lyrics for different languages, as again this would likely affect the spacing.
Elements in layers could (and should) influence collision avoidance and all aspects of spacing anyway. Just like when you switch on cues, harp pedals or multirests. I can see several possible use cases, like separate layers for Urtext and editorial bowings or fingerings. Inevitably, that would change the rest of the layout. I think some layers might need to be mutually exclusive.