Legitimate? Basically none. Illegitimate? First, lazily fixing a fuckup on putting up strings of Christmas lights where you can't daisy chain them properly, with bonus points for the likeliehood of needing to break off the grounding pin. Second, injecting power from a generator into a single circuit of your house if the power is out.

In one sense, you could argue conductors are conductors and if you think through every eventuality you can mitigate risk, but on the other, if you find you're in a situation where one of these seems useful, you are not the type of person thinks through every eventuality.