it is a lil weird that only nicole is the one singen but did you know that she had a solo deal b4 she joined pcd? she only joined them for one album, but since her album didn't work out she decided to come back to them. all of the girls CAN sing, really, the only one i'm not sure about is kimberly I've never really heard her sing but for rest they all sing, ofcourse Melody's voice is stronger than the oters. so i can't see why they woudn't change it up a bit and have more of the girls sing

And I think Melody or any other girls won't up and leave just like that because we all now how hard it is for artists these days, even with all the talent and beauty and starpower they can still flop. labels are not willing to take big chances on 'new' artists, so the girls might think it's saver to stay with pcd.