Not sure I want to develop on an OS that automatically applies updates - keeping a stable environment is important. I would fear that some dependency could change and break what I'm working on without notice.
While I agree (I disable autoupdating on all the applications that I can, and find Firefox's insistent need to update for security patches annoying), so many people use auto-update for their local apps, and mobile devices, and external services like GMail, that you and I might be a minority.
CoreOS offers three channels: alpha, beta, and stable. The idea is that when you deploy a CoreOS cluster, you run mostly stable instances but mix in some alpha/beta instances as well. Automated updates always propagate from alpha to beta to stable, so if an auto-update would break your application(s) you would notice this on an alpha instance before it would hit your stable instances.