They make Soyeon sing most of the linse because they want to not because they have to!!
Hyomin can sing very well and so does Eunjung!
The others may not have the best vocals out there but they're still very good!!
They don't give too much lines to Boram, Qri and Jiyeon already .. if they add two more members with more powerfull vocals how many lines will they have now??