Yeah, in low-fraud scenarios it's a very good idea. Otherwise, though, you have the problem of what happens when a robber takes it.

I'm thinking a ring type device might be better--put a pulse oximeter into it, you unlock it with your phone, it remains unlocked only so long as it gets basically perfect data from the oximeter, locks if it fails for a second. Thus said robber can neither snatch your ring nor cut off your finger and use it. I like the metal mesh straps that can hold my device very snugly against my skin without being tight and that would be good enough, but a looser strap would not.

The smartwatches I've owned with payments support (Pixel Watch series) automatically lock when they are not worn, presumably using the heart-rate sensor.