for sure that you don't do it on coasters!
I could probably count on one hand the number of people I've seen in recent trips with
one of these, though.
People by and large don't use them. Pretty sure none of us would enjoy the experience of a loose cell phone hitting us in the head on ToT -- I don't think you need the acceleration of RnR to make that dangerous.
I wouldn't be surprised if we DO see more rules on this in the future. Selfie sticks "encouraged" people to do this more, and they pretty promptly banned those. So many seem obsessed with filming or live-blogging their entire lives these days. Not sure it's going to get any better.