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Nope (almost never), which is why GNU rm requires the '--no-preserve-root' flag if you actually want to do that for some reason.

In the manpage for rm, I see "--no-preserve-root do not treat ‘/’ specially (the default)". For real? The default is to do the worst thing?

No, the default is to do nothing. The (default) refers to "treat root specially" not the flag.

What are the almost never situations?

I'd be super interested, because I cannot think of one at all :D

Cops / assailant about to bust down your door and its all you know / have time for?