procmail+spamassassin is an example of a long-lived daemon and a client that talks to it, but indeed, spamassassin had to provide both parts because procmail doesn't directly do that.

I have 20 year old code that does the same client/server thing, by having procmail leave files in a directory that the daemon is watching for new files to appear in.

So I'd agree that it's a useful piece of sugar!