The reason they wanted to use a female voice in the second number is because it is a different timbre, the original is Imogen Heap. Why not use the source material to give great emotion and sonority?
String Instruments, again in the ballad are a way to bring a different emotion and sound to the piece.
BTW: the tuba solo is full of electronic call and response with the tubas. A lot of A and B there.
Go Blooooooo