Not sure about the first part of your comment as neither RHEL nor CentOS use systemd right now (in fact RHEL 6/CentOS 6 uses upstart).
RHEL 7 will be systemd based, but it isn't ready (yet), and in my experience people tend to stay in the major release they're in instead of upgrading. So it will be a long time before systemd is really widespread between RHEL/CentOS users.
RHEL6 was released in 2010. That's why they are "still" using Upstart, because systemd didn't exist when RHEL6 was being qualified. Fedora (which is upstream/testing for RHEL/CentOS) switched to systemd years ago. 3 releases ago at least. RHEL is the equivalent of what Fedora was a few years ago, plus testing and bugfixing by people that get paid from the billions in support contract money that Red Hat pulls in every year. RHEL7 betas and rcs have been available to server admins under contract for a while, they are likely figuring out upgrade issues currently, and when it comes out they probably will switch over a lot of their machines. All those people have support contracts with Red Hat, the packages are qualified and if something goes wrong they get to call up and complain. If you are coming from the Ubuntu world RHEL is Long Term Stable, and upgrading to a new LTS might be work for your admins but the OS should be solid already.
RHEL 7 will be systemd based, but it isn't ready (yet), and in my experience people tend to stay in the major release they're in instead of upgrading. So it will be a long time before systemd is really widespread between RHEL/CentOS users.