It's typical practice for major league teams to give some left-handed batters a rest when the opposition starts the game with a left-handed pitcher on the mound. This, of course, is because of the well-known platoon advantage that pitchers have against same-handed batters. I remember a time when it was rare to see any left-handed batter in the lineup against Randy Johnson, but we don't seem to have a monster like that in the game today. Most managers will leave their best lefthanded bats in to face a lefty starter, with the more marginal lefty bats replaced by right-handed hitters.
But not all pitchers are created equal, especially when it comes to this theoretical platoon edge.
One thing to note is that left-handed starters tend to have a smaller platoon differential than left-handed pitchers at large. I believe this is because managers are aware that the opposing team can stack their lineup with righties and tend to select left-handers with a smaller platoon split to be their starters.
Makes sense, I think. Here is a table of the current left-handed starting pitchers in American League rotations. I count 20 of them (I don't include those on disabled lists):
|2009 to 2012||wOPS|
|* BF is approximated here as AB+W+HBP|
|wOPS is (1.75*OBP+SLG)/4|