Earlier quoted context omitted.
The dependency caching is orthogonal to bottling; it's just that there's little incentive for me or anyone else to set up or document the bottling process (for ROS on OS X) until the created bottles are usable, which is not presently the case.
The bottles are usable? Homebrew and some other third parties already use bottles. If they aren't working for you: please create an issue so we can fix them. Thanks!
There's no point in the ROS community bottling ROS packages for Mac-based robotics work until it's possible to install them without a long dependency-resolution step. That's the usability issue.