I've never found another players ball on a WGT green distracting. Maybe its just a matter of attitude. I know that the other ball isn't really there and won't deflect my putt.
Real golf only requires a ball be marked when it might interfere with a players line of putt.
As a point of golf etiquette a player may request another player to mark a ball, but most golfers mark their ball so they can pick it up, examine it for defects, and clean it.
Back in the day a player could find his ball 'stymied' with an opponents ball between his ball and the hole. The opponent was not required to mark and lift his ball.