For me it doesn’t matter if they’re accurate, only that they’re consistent. So if you walk 20 steps and it counts 12, then you walk 20 more and it counts 12 more, that’s useful. If your daily total is 5,000, and tomorrow it’s 10,000, you know you were twice as active, even if you didn’t really do 5,000 or 10,000 steps.
I don’t think knowing exactly how many steps you do is useful, but measuring your activity is.
If your counter is inconsistent, so sometimes you walk and it counts too few steps, and sometimes it counts too many, and sometimes it’s right, then that’s useless.
Also bear in mind it will count steps that aren’t really steps. If you do some sort of activity where you shake your hand up and down for several minutes, it will count them as steps. That’s assuming you’re wearing a wrist strap.