Because with singing, you can at least tell some kind of story. (Even though it's repetitive... breaking into a 5 minute song that says the same thing over and over again instead of a 1 or 2 minute conversation that isn't reptitive.) With dance routines, what the hell story is even being told?Nah. Why would one be more over the line than the other?
"Subspace Rhapsody" is one of the only episodes in the franchise where I kept looking at the remaining time just because I wanted it to be over soon.