I used it a couple weeks ago when I just wanted to replace a few values in a template file in an automated way. I used sed at first until the number of values involved made it awkward. I certainly didn’t want to bother with jinja or mustache etc for this trivial little case. M4 was there and handy.
But I’d rather swallow my own tongue than use it to configure Sendmail again. Postfix had many compelling features that made me want to switch to it. None were as insta-buy as comparing its configuration language to Sendmail’s.