If you follow that reasoning, no person is useful and no tool is useful.
The little box I'm filling now is, compared to a lot of other interfaces, a shitty interface. That doesn't mean it isn't useful. Probably it is getting replaced, only with a slightly better inferface.
The karma system is quite simplistic and far from perfect. I'm sure there are ways to go around it. The moderators make mistakes.
That doesn't mean the karma and moderation are not useful. I hope you get my point but it's fine if we disagree as well.