I currently live in the American South, where every English speaker within a hundred miles speaks with a rhotic accent. Recently, I spent an afternoon with a local choir in practice, and was somewhat perturbed by the instructions given by the director to drop Rs and modify vowels to sound more British — non-rhotic, specifically. I've spent a fair amount of time over the last few years learning some of the finer points of English (especially for second-language learners), so this smacks of trying to teach someone a prestige dialect. Most of my (formal) musical experience has been on the violin, so is there a specific musical reason non-rhotic accents are preferable for singing, or is it just a matter of this person's arbitrary taste based on which dialects currently sound impressive, high-class, and fancy?
Poking around online suggests it may have something to do with certain American vowels constricting airflow slightly; how significant is this effect, and how does this connect to rhoticity?