I have heard a rather... consistent case made for the idea that Pinkie masterminded the whole thing to show Fluttershy that singing on stage wasn't so bad. She voiced Fluttershy's irrational fears, orchestrated Big Mac's voice problem, knew Fluttershy would have to fill in, and after Fluttershy was exposed, she (quite painfully) demonstrated that even Fluttershy's worst fears weren't really all that bad in the end.
Personally I don't buy such ad hoc justifications, but as long as we're throwing these theories out there, this one should be on the table.