[Kelric]

One second in major league baseball of a runner running down the line with a lefty infielder is the difference between an out with a righty infielder and a basehit. The mechanics are too slow and awkward for them to be able to do do the job in the bigs unless they are exceptionally talented. Imagine a second baseman, throwing righthanded, taking a throw from a shortstop on a double play. He can catch the feed and throw the ball in one motion without ever having to move his body too much. A lefty is going to have to catch the ball and spin in one direction or another to make the same throw. That time difference against a decent runner means no double play.

Now as for lefty catchers, aside from maybe a slight and almost innoticeable difference in throwing guys out, I think they can play without a problem. Throwing around LH batters for a righty catcher never seems to cause too much problem after all.

Edit - As for lefty catchers throwing to third as tysok (who was typing his reply as I typed mine) brought up, I don't think it would be that bad. You see righty catchers throw behind a hitter to first base once in a while without ever getting up. Lefties could do the same behind the plate. Sure it'll be a bit slower on the throw, but weak armed right handed catchers are in the bigs after all.