Gary ... while you and I are most often 'on the same page' about things, I have to disagree with you on a couple of points ...
First, when you talk about 'jazz' singers I 'assume' you are talking about people like Sinatra, Bennet, Ella, Torme and more recently, Michael Buble and Diana Kraal - people who sing primarily from the "Great American Songbook" - ...
I do not agree that these vocalists are "trying to hit notes out of their comfortable vocal range", not by a long shot ... but knowing how to effectively use a hand held mic, they are making sure that the power note they hit does not cause distortion by the mic being too close ...
Secondly, while it is true that many, if not most, of today's pop or country performers use headset mics, one major reason is the amount of 'choreography' that goes into their performances - yes even people like Garth - very little of what happens on stage is NOT well rehearsed, even the seemingly 'ad lib' or spontaneous happenings ...
AND working with every one of these performers is a well trained sound man at the mixer, who knows EXACTLY when to boost or lower the feed from the 'star's' mic.

Microphones, like politics, religion, and keyboards are chosen by individual needs and tastes ... you and others obviously do VERY well with a headset, while Donny and others do VERY well with a hand held ... that's why they make and sell BOTH ...
JMHO ...
BTW ... if I am wrong with my assumption above, I beg your forgiveness ...

PS Scott... sorry if this thread goes in the wrong direction ...