With the advent of xhr, more and more large code bases are finding their way onto the browser. Now days, you can find applications with a million lines of code running in the browser. And, as browsers get compliant to HTML5 this trend will only continue, the future looks bright.
The Closure Library (not the compiler) makes it easier to use the Closure Compiler. However, if you are not using the Closure Library there are a few things you need to set up to get your environment nice and productive. For example, you'll want to be able to develop directly from the source files for save and reload development. You also will need to be able to debug the compiled version via source maps. If you are interested in trying this type of development I built a kickstart project on github. It only requires Node and comes with a simple example project to get you started.
I find it convenient for its simplicity. Just point it to your source files and keep using your favorite text editor.
Even using Closure Compiler is a good example of why you might want to avoid supersets. Any of the supersets can benefit from the Closure Compiler, but can they use it? Probably not, or with limited functionality. Usually, you will have to wait for someone or some third party to make it possible, and yet another dependency and less flexibility.
Mahalo for reading!