No shit, Sherlock! :lol: Since different disciplines involve requiring completely different things from the horse, it pretty much follows that the types of equipment used are not likely to be the same in all disciplines. For example pretty much the last thing you want on a racehorse is a gag yet you own't see a polo pony in anything else. In a similar fashion pelhams, kimblewicks, double bridles and the likes with a curb action are also not wanted on racehorses but you often don't see many showjumpers or dressage horses without one of the above - or a gag - in their mouth.
Incidentally dexter ring bits were originally used for steering purposes more to keep horses straight if they had a tendency to hang or lean, were difficult to steer or used on unpredictable types for similar reasons. Nowadays they tend to be used for pullers more than anything else. The only two horses I've had dealings with who were ridden in ring bits had them as the first one could hang and was strong as well; the second one had a very bad habit of charging off the side of the gallops or whipping around whilst cantering.