RCS is a pretty excellent tool when you have a single file (like httpd.conf, hosts, a zone file) that you want to manage on a server. 80%+ sysadmins know how to ci/co and use rcs in that context. Likely less than 10% of sysadmins would grok the use of git in that context.
Also - does git even have the concept of "check out with lock, check in free lock, etc?"
Citation needed, because my experience is the exact opposite.
Also, your experience is probably not exactly the same as 80% of sysadmins. But I don't have a citation for that.
For server based control, it's mostly CVS, and, more recently (last 10 years or so) subversion.
For local file control it's all rcs .
It's what I learned in College, saw again at Netscape, again at Loudcloud/Opsware (though, for source control of code, they used Clearcase of all things. Shudder.) - always for the purpose of managing a single file. Once there was more than one file, something else was typically used.
There are a lot of sysadmins out there. Most of them are older than 25, and most of them do not work in Silicon Valley. They work on all kinds of systems, for all kinds of companies. Any company that's been in operation for more than 10 years has files managed by CVS, or RCS if they've been around longer. They probably also use other tools and systems you've never heard of.
But that's OK. Today you'll get hired to manage new systems with new tools. In years to come, somebody will tell you how nobody uses SVN or Git anymore, and they've never heard of anyone that does.
...although it wasn't ideal for me, and it doesn't look like it's been updated in a while. E.g., these days that should be on GitHub!
RCS really fits well with the use-case of /etc configuration files. It's a pity that Git doesn't function very well for that particular case.