Somebody: If it's merely a matter of appropriateness of the metaphor, then we can just as well say that other mammals aren't hard-wired for circadian rhythms (or anything else) either. If wiring necessarily invokes images of computers and binary code, then it wouldn't apply for humans, ocelots, or bandicoots. And then the strict distinction that was supposed to be made between humans and other species vanishes.
Personally, I never used the phrase literally, but rather to express that this aspect of our behavior has a physiological basis and is not primarily a cultural or personal convention.