why not. 1.if your harmonies do not fit with the booked guest artists.2 if you are tone deaf.3 if you have no sense of rhythm.4 if you are completely drunk and unable to walk straight, you might think you can still sing in harmony   but probably cant.
to kitty,
this talk of modulating into harmony alternatives, might be alright but modulating generally refers to changing keys,could be dodgy, if the other singer stays in the original key and you modulate to another.