I gotta agree with the previous poster...

The point of being called a group is that everyone sings on every track.

Take Destiny's child for example - Beyonce did the lead vocals but Kelly always got a few verses in and michelle would do the Bridge.

There's nothing wrong with Nicole doing LEAD vocals... as long as the other girls get featured on every track aswell.

It does surprise me though that melody is not on joint lead vocals, it'd make the songs more interresting.