I don't understand why some people think UEE is more useless than Juyeon? I'm not saying that either are, but in the grand scheme of things, UEE is better at everything than Juyeon yet people really like Juyeon for whatever reason.
And don't with Sulli, she's not a great singer by any means but her voice is really pretty. Dynamically, she contributes a ton to f(x)'s tracks.