This seemed underwhelming at first, especially since Zoey Samaras, who should benefit from this the most, finds it competing with her signature, and Relic Hunter feels like a heavy investment for something like this. But the more I think about it, the better it seems. Any turn where you make an engage action is one you'll be fighting, and any turn you're fighting will likely benefit from one more fight action, whether it's just in case you miss on the first, or whether the target needs multiple fights to take care of. Even if you don't end up using the spare action to fight, it can get a resource or a draw.
Inside of Zoey Samaras, it might be worth picking with intentions to upgrade to Relic Hunter - with it and her cross in play, not only can any one health enemy be killed instantly, any one health aloof enemy can be killed without tests or real actions.
Outside of her, there aren't many Guardian cards competing for the accessory slot. Hallowed Mirror is nice, but can only be taken at 1 copy, so is less likely to be drawn, and can be used up and replaced by any other accessories you might be using. Empty Vessel is similarly limited, and costs a significant chunk of XP. Unlike the mirror, it stays in play, though. Police Badge is also not a level 0 card, but it can be discarded for the whistle if you draw it, though I'm not sure what kind of situations that would be good in.
As a cross-class option it's less appealing - Mystics, and Innsmouth's own Sister Mary prefer their -boosting accessories, Survivors have cheap sanity soak that Yorick and Tommy really don't want to lose, and Seekers have plenty of good options, though Roland and Joe might give the whistle a look, depending on the build. It's probably worth looking at for people with a Rogue/Guardian pool, though - it can save a shot on a Colt Vest Pocket, or add a resource to a Pay Day turn, for instance.