> But as a composer I most certainly wouldn't want string players choosing their temperament based on whether there happened to be other instruments in the ensemble capable of the same.
This is a weird way of looking at it. String players aren't sitting there consciously thinking of their tuning as they play - they're doing it by ear in real-time. The tuning they use will be the one that best harmonizes with the other notes being played at that moment.
> And it sounds off for music that doesn't largely sit in a single key signature anyway,
That's actually where the ability to adapt tuning dynamically is the most powerful - it allows you to be in tune relative to other pitches being played in that moment, not just in tune relative to some absolute benchmark that nobody is going to be able to hear anyway (because almost nobody has perfect absolute pitch).
Sure, I imagine it's not dissimilar to how we sing as choristers.
But I've played on keyboards tuned to exact just temperament in a particular key and it starts to sound very weird very quickly the moment you veer off the reference key signature.
> But I've played on keyboards tuned to exact just temperament in a particular key
Well, that's your problem. You're using a keyboard, which doesn't permit you to harmonize dynamically the way an unfretted string instrument does.
Even within a particular key, the pitch that sounds the best for a particular note will depend on which other notes within that key you're attempting to harmonize with. A keyboard can't do that.
Btw, this is from the wikipedia article on Equal Temperament, and I'd say it aligns with my general understanding/ expectation:
"Unfretted string ensembles, which can adjust the tuning of all notes except for open strings, and vocal groups, who have no mechanical tuning limitations, sometimes use a tuning much closer to just intonation for acoustic reasons. Other instruments, such as some wind, keyboard, and fretted instruments, often only approximate equal temperament, where technical limitations prevent exact tunings.[4]"
No and it's possible that as a pianist my ears are more attuned to prefer equal temperament than those of a string player. But I admit when singing a capella there are occasions particular chords just seem to sit better than when having to match a piano accompaniment, and to some extent that's likely to be the ability to use "purer" intervals.
This is a weird way of looking at it. String players aren't sitting there consciously thinking of their tuning as they play - they're doing it by ear in real-time. The tuning they use will be the one that best harmonizes with the other notes being played at that moment.
> And it sounds off for music that doesn't largely sit in a single key signature anyway,
That's actually where the ability to adapt tuning dynamically is the most powerful - it allows you to be in tune relative to other pitches being played in that moment, not just in tune relative to some absolute benchmark that nobody is going to be able to hear anyway (because almost nobody has perfect absolute pitch).