lol, I didn't even really notice it for Nana Korobi Ya Oki since the costumes were the same, and the "dancers" whispered stuff so while I noticed that they weren't singing much, I didn't realize that they were not singing at all.
Also for MM'15 it was more explicit because of how the back girls got to lead (well, minus Eripon... which I wasn't happy about >.>).