Hi Luke
My only thoughts are that if the boat capsized and you are still attached to the rail, if your tether is not sufficiently long enough you will be held under water!
Clipping onto side rails and the same thing happens your clip may well be only a foot or two under water but you are on the surface.
My concern would be that if the boat went down you could go with it, not nice
As far as I am aware fast yachts use the safety line for when crew are moving about on deck, if you are on your own I would never venture forward or going down the sides unless it was really calm and there was a real need to. Most jobs can be carried out in the main deck area without the need to put yourself in danger.
Just my thoughts
Dave