Well, it's something we should look at, especially the library makers.
People who write ClojureScript regularly encounter this: we get this optimized and trimmed down app, and then something needs jQuery, so we have to pull all of it in. After you've used advanced compilation for a while, it feels downright dirty and wasteful to pull in entire blobs of code, not just the function trees you actually need.
You can also define a separate externs file with a list of symbols that shouldn't get mangled; this is useful if eg. you're using a third-party library that you don't want to modify.
Public methods get unmangled symbols. Everything else gets renamed to a short name to save bandwidth.
Dependencies are specified with goog.require.
Anything that doesn't get required with goog.require or isn't called by a public function gets culled.