I think we need to take into account that there are multiple different purposes for a minimal image. If I'm using the minimal image in Docker, I don't want or need sshd. In other (non-container) places, I probably do. So maybe we need more than one minimal (or micro) -- 'micro' (outrageously small image for outrageously small machines) and 'micro-lxc' (even smaller image for even smaller containers).
I definitely think yum should be included, though. I should be able to 'RUN yum -y install ...' in my Dockerfile without first invoking some ugly 'RUN rpm -hvU http://...' mess.
One of the many cool things about containers is that we can really almost eliminate all of the userspace except for the application itself. With that in mind, including yum is probably a violation of Twelve Factor (specifically,
http://12factor.net/build-release-run), but the Docker build process doesn't really permit truly strict separation of build and run toolsets, just of the stages. As LXC tools continue to evolve, though, there may be a use case for a micro image without even yum, with the assumption that the build stage will be accomplished by an external toolset. That'll be an opportunity to really experiment with how much userspace we can pull out of CentOS.