I've personally never done a straight play with a mic, and I did one musical without one. I'm fairly certain that for South Pacific, at least through previews, Paulo Szot was not miked.
Now I see in a NYT article that miking apparently reduces the strain of speaking:
"Surprisingly for a singer trained in opera, Mr. Szot has called the body microphone a 'wonderful thing' that enables him to deliver spoken dialogue with conversational naturalness.
"Just one week after opening night, he was afflicted with a throat ailment and had to miss several performances. In interviews he said the strain of speaking his lines, something he was not used to, was the cause of his troubles."
p.s.
Link