My take on the "breaking changes" issue is that there should be at least some APIs people can trust to be stable. A library in development could have both an unstable version of the library and a stable version that calls into the unstable one. (This would be more practical if Arc had namespaces.)
Of course, if you don't know what features people are going to find useful, it's not especially possible to design a stable API. It's an approach that makes more sense for code that's designed to be something in particular, rather than an experiment.
