Ah, but that's about cryptosystems. And its application is really about the continued use of a thing. Kerckhoff's principle doesn't apply so much to a one-off situation. Sticking with a house raid:

We have a plan to raid this house using these entrances, covering those exits with these 10 people. The adversary knowing our plan screws us, but learning it after the fact doesn't help our adversary.

But what if we use the same tactic every time? What if we always cut power immediately before breaching. That's a signal to the adversary. What if we always use the same sniper and he has a known handicap? If the adversary learns these things then they can break our efforts (or reduce the effectiveness) to conduct the raid. And this is where Kerckhoff's principle could be applied.

EDIT: It's not secrets that are the problem, it's keeping the wrong secrets. You can't have my passphrase or my PGP private key(s), but, sure, you can have the AES and RSA algorithms and my public key(s).

Sometimes a fragile plan is still the best one you can have. Life doesn't always give you good choices. Sometimes you're stuck with either a fragile plan where things can go horribly wrong if secrecy is breached, and one that's even worse.