The use case where this has a significant impact is not for package authors but for projects that follow package based architecture, and even more importantly, isomorphic end-to-end packages. Since folks have been splitting their projects into isomorphic packages, something which I think the Meteor community pioneered, then I’d argue that the speed and behavioural consistency of the rebuilds during development within the packages is as important as the development in the main project.
But I’d agree that technically this is not breaking change since the behaviour was rather assumed. Nevertheless, given it’s demonstrated usefulness, I’m hoping we can get something official back, I’m personally hesitant to update due to this change.