It's not necessarily that SC is complicated, but it is a custom language that has to be learned to use a single application. Also, many musical ideas and projects extend beyond what sclang can do, or you want to use external libs, etc. By using Clojure you get all of the power of sclang and more, because you also get lisp macros, while also getting access to the whole java ecosystem of libs. We've got a gui library for Overtone in the works, and it will be a good example of this in action.