It's funny that you used NYCB as an example. I've seen some of my favorite performances there (much like on broadway) done by non-principals being put into leading roles. Once upon a time, I got to see then-soloist Robert Fairchild play Romeo in R&J. It's still one of my favorite performances.
Similarly, I've seen some of my favorite Broadway performances come from understudies. Recently - Alysha Deslorieux as Eliza. I know this will be unpopular around here, but I thought she was far superior to Philippa Soo.
That being said, I do think the amount of understudies I saw last week was a bit odd. There were understudies for Eliza, Mulligan/Madison, Lafayette/Jefferson (obviously), Phillip Hamilton, James Reynolds/Phillip Schuyler, and obviously all of the respective ensemble tracks.
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