I posted a Dara post earlier, and I remember that Dara always cried because she said she wasn't good at anything. When I saw 2ne1 tv, Bom tried consoling her, but she mentions it to this day. I know that she wasn't like the most talented or whatever but I actually don't think that's necessary.
If every member is amazing at something, sometimes that diminishes the overall effect of the group...as confusing as it sounds.
And for me I liked Dara, not really because she was pretty (I found Minzy pretty too) but because she was the most active with fans.
Yeri got a lot of hate at first, and even now. But we're all just hypothetically speaking when people say Red Velvet don't need Yeri. Dara was one of the most popular members and she felt like that. Poor Yeri got all the hate.
Warning: Eating Disorder
Now OMG. Oh My Girl, is a girl group you all probably know. But what you didn't know is that there was another member, JinE. JinE trained the most, yet with Arin was given the least lines, and also had a lot of haters for her visual. She eventually developed severe anorexia and had to leave. She trained for so long...
Now Shuhua, back on the old forum did get threads saying that she was useless. I am not a Nevie so idk how it is like for her now, but she does seem to be good at variety, and kinda like Yeri, seems to be very social and open.
I KNOW all the Orbits on the old forum were super worried about BBC giving some Loona members very little lines and promo in their groups, and I remember Yeojin(?) not getting a lot of spotlight and another member (VIVI??).
I think one more person was Jihyun from 4minute, but I don't think she really got hated, but she never really had stage presence compared to other members like Gayoon and Hyuna, which in a group like that, makes her stick out.
So, remember that if you think that an idol is useless, they probably know and think it too. Nevertheless they are there and we should support them.
TLDR: People's arguments make no sense