I feel the same way about it, Munk. It particularly bothers me in Jerry Herman musicals--though I think it's done in a different way though it still irks me to no end.
MACK & MABEL is one of my favorite shows but I can't stand it when Herman uses his formulaic 30-people-ensemble numbers like "When Mabel Comes in the Room." Same thing with "Mame," I like the score but hate that one number--and yes, I know it's a classic and whatnot.
This also happens in SHOW BOAT during "Can't Help Lovin' Dat Man of Mine" after Julie does her beautiful rendition and Queenie and Joe do theirs, the whole ensemble starts singing and it just irritates me.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"