OK, while I agree this is a risky thing to do, it's not the same as offering your child up to predators.
Parent posts child's real name, photo, school, login details on public site advertising for children to join up groups, and then leaves child to it = horrendously naive and dangerous, even stupid.
But just exchanging online names, on a game you supervise usage of, keeping dialogue open with your DC about online safety and monitoring their actions and being there to back them up and support them if they see anything they aren't sure about, that could actually be a very positive way to introduce DC to the world of online gaming and safety issues contained therein.
I wouldn't personally use such a site but I don't think it's as inherently terrible as you're making out. That's like saying dating sites are awfully dangerous because rapists use them. Well, of course they do, but lots of genuine people use them as well.
If you just lock your DC away from anything potentially dodgy and don't allow them any freedom to navigate situations themselves - with full backup - when they do encounter these experiences they will be totally naive and possibly more at risk than a younger child.
Just as you hold a small child's hand to cross the road and as they get older you slowly allow them more freedom, you wouldn't carry them across every road until they were 10 and then let them out alone, they'd get killed. Children today are going to have to navigate this world as adults, it's our responsibility to educate them about it, not protect them from it.