Well if you don't like Dart, then this obviously doesn't mean much to you. If you do like Dart and have existing Angular projects, then you'll have an easier path to migrating away from JS.
> What's the value of this, I use Angular a lot and love it, but Dart only adds a degree of complexity that I don't want to debug with.
Then it has no value to you.
Other people find Dart to offer more over JS than that for them, and for people for whom that is true, Angular-on-Dart offers advantages over both Dart-without-Angular and Angular-on-JS.
During development, you can just use Dartium (a special build of Chromium with native Dart support). With Dartium, you can debug directly from within the IDE. There are also source maps if you ever need to debug it elsewhere.
As a Dart user I can tell you, it feels more right than JavaScript with true classes, interfaces libraries, debugger, etc.