Coski and Seg come on. you encourage people to shrug the whole thing off. you are cleverer than that. to say a signal is useless because there is one instance in 50 where it is in-effective is poor logic. I apologize for criticizing, but really. It works in cycling, it works in rock climbing. I have never met MR, but it is no co-incidence that we both use the exact same signals to announce our intentions with respect to line, direction change, slowing or stopping completely.
And for the most part,others get the intent of the signal.
A mistake cyclist make is to assume that they are so strong that no one could possibly be passing them and that they are free to be everywhere on the road. Same in skiing, but I can tell you, at most mountains, there are people who may be overtaking you, in control or not, thus the usefulness of a signal.