Javscript is an implementation detail: this is the same old Google trap.

Benchmarking has shown a very real difference between gzip alone and minification+gzip. This is especially true given that most browsers will block parsing and loading of further content in the page, and all browsers will block rendering, while waiting for a JavaScript include to download. (They have to block at least rendering in case it does some document.write()s, e.g., ad JavaScript.)

You can't get the same effect with just gzip. There's a ton of irrelevant stuff that gzip can't possibly compress away -- comments, for instance.

I suggest the O'Reilly book "High Performance Web Sites" for more info about this kind of thing. It does include benchmarks for minification.