I use svn:externals to add references to plugins and themes directly from the WordPress.org repository, in order to not have to maintain them in my own code repo. Every time there's an update, I have to go back and point the externals to new versions, which is a pain, especially with trusted themes and plugins, which I'll update without even thinking they'll break my sites.
Now, it would be much easier, if I could point the external references to a magical "latest-stable" branch, which will always contain whatever is the latest stable version of the plugin, theme, or ever core! Similar to how latest.tar.gz works for core. That way, I can set everything to latest stable, and simply run svn up every once in a while, without having to go back and forth looking up latest versions of the thirty plugins I need to update.
Please don't confuse this with trunk. Trunk is where development happens. Latest-stable is where a milestone has been reached, and a release has been made.