I dunno McGrath, all the singers I've ever known weren't trained by a PA system, but by other singers. It would be great if everyone sang out--maybe--but everyone doesn't. I've heard some very good singing through a PA system that I wouldn't have heard at all, otherwise, cause the singers didn't project much--but were well worth hearing. And I fully agree with the need for balance as an argument for modest amplification. Sometimes it is necessary so that everyone can be heard.

And (my bias showing here) as the sometime sound tech in a rather small room, it's part of the setup to decide if amplification is appropriate. One rule of thumb is that if alcohol is being served, PA is always appropriate. Nothing more frustrating than amateur performers who are afraid of microphones getting up and blowing their lungs out, in a moderately noisy room, without being comprehended. You can see them making noise up there, but it's not getting across. Shhhh-ing an Oktoberfest is not a solution that works.   

Having said that, I quite agree that those big old echo-ey halls are perfect for using the natural acoustics. On the other hand, the Unitarian church in Ottawa, built in the 60's, is a very nice environment which was designed to be visually and acoustically pleasing with the built-in PA--and has become a favoured venue for folk music concerts because of this.

Best
W-O