We rely on the API boundaries between the kubelet and the kubernetes API server remaining well versioned without backwards compatible changes. The k8s project is very conscious of the needs for that and work hard to ensure that there are no breaking changes between releases. This is similar to all pieces of software in a CoreOS release: systemd, docker, cloudinit, Linux, rkt, etc.
I see. I suppose the only down side of using the CoreOS version is having to wait for it to be updated when new minor releases come out with new features/improvements.
