Except there are plenty of players that hit both righties and lefties well, and some players (Cedeno) that have reverse splits? Also most teams don't have the roster space to field enough players to do it. Its great in certain situations with exaggerated splits like Johnson and Edmonds, but the rest of our players mash lefties and righties equally. (Or at least our offense as a whole has anyway, this isn't the 2007 Cubs who flailed uselessly against lefties most of the year)