That's why their songs end up being messy either in terms of line distribution or in quality.
Like with their first two title songs, Eleven and Love Dive, the quality of line distribution was pretty great. But some members were getting less lines. Whereas with their later releases they try to make the line distribution more fair and it ends up hurting the quality.
Ideally, for the songs that they choose, Rei should be doing all the rap parts and the rest should be divided between the other 5. But since that would make her lines the longest in the group they just decided to cut the rap parts in half and make a space for Gaeul, who is just not built for that.
Imagine what a mess it would be if half of Soyeon's rap in Idle's songs would be given to Shuhua. It would be ridiculous and I am glad Idle are not prioritizing equal line distribution. Yes, that leads to a loud minority of fans bitching about it on Twitter, but who cares? If they want fairness they should choose songs with very little rap parts.