I have done it years ago, at shows, and I did it to disguise the fact that it was different water to what they got at home. Because water in different locations could have a different taste.
However haven't used it since we moved here, as goats have drunk much better we moved here. Plus, now I would probably use molasses to tempt them to drink rather than cider vinegar.
I have a vague recollection about cider vinegar being good at mating time, but I can't remember why now....