Today, systemd is included with our 7.2 and newer base images. We are putting the finishing touches on the work Colin started earlier this year and plan to release a new, minimal base image. I've been toying with the name rhel7-core, but that name sucks and will likely change. Since the new minimal image will contain a minimal package manager, I don't want to promote this one to be something like "rhel7", and change the current base image to "rhel7-systemd", or other. That would be too disruptive IMO.
I don't see changing the default CMD to start systemd as being problematic, but I don't see it as very advantageous either. It's trivial to add CMD ["/sbin/init"] to dockerfile to use systemd, and **nothing** breaks for anyone. I'm leaning towards the opt-in model versus opt-out. Anyone want to convince me otherwise? :)