Absolutely agreed, Will. Listeners grow very tired if the key does not change.

Of course one has no choice if the policy is to tolerate (largely) diatonic instruments, or chords from a limited free-reed-"basses" preset. But in all other cases, the result is well worth the additional effort.

Guitarists can use capos. As for fiddlers, the problem with flat keys is not the first position as such, but players who heavily rely on the use of empty strings. This is generally discouraged anyway, because the sounds differ from the ones of fingered notes. (There are exceptions to this rule, of course.) Thus, flat keys provide a good training.

If singers are involved, trained or untrained, they should be allowed to determine the key freely, or at least up to a semitone.