I know that unpires will replace fouled balls-- either those that were hit out of fair territory or those that have presumably been scuffed by making contact with the dirt around home plate. This is-presumably-- to prevent pitchers from using the scuffs to their advantage. Why then do they not replace balls after every play? Surely the damage inflicted by a hard ground ball to third base, either by the bat itself or by the infield dirt, would be as much as they inflicted by a 58 foot curveball. Not that I've lost sleep over this or anythng, but was just curious.