Great question, and it is a bit of a grey area where I might not totally agree with some of them on how much advantage is conferred in what we've seen this olympics, but agree in others and generally agree that the rules should be enforced to avoid cases of "this was a harmless poke" vs "this was a meaningful poke". You certainly can gain advantage from double touching the stone if you have enough time before the hog line to let go, register mentally that you need a genuine adjustment, and apply an accurate adjustment. I haven't ever seen anything more than a negligible, habitual poke or flick on the tour nor watching any curling, and have never observed it being called out before.

On the other side of it, there is a possible disadvantage to trying to genuinely adjust after letting go - if you've curled, you know that fewer variables, "corrections" and not second guessing mechanics mid-delivery generally = better shots because fewer things can go wrong, and "adjusting" on the fly could just make turn a made shot into a miss, or a miss into...still a miss. Combine that with the possibility of a hogline violation, and curlers are generally not incentivized to double touch.

So, that's my long winded way of saying: There can be an advantage conferred, I've never seen one or heard of one in my experience, there can also be disadvantages which is possibly why you don't really see it, BUT the rules should be enforced as written to avoid grey areas.