In MLB, if a Catcher drops or misses the third pitched strike, the batter becomes a runner and can attempt to reach first base before tagged out or forced out at first.
6.09 The batter becomes a runner when -- (a) He hits a fair ball; (b) The third strike called by the umpire is not caught, providing (1)
first base is unoccupied, or (2) first base is occupied with two out;
-- Official MLB Rules
What is the reasoning behind this rule? Why does it only happen if first base is unoccupied, and what's with that crazy extra exception of two outs?
The exception about first base being unoccupied is to prevent a catcher from intentionally dropping a third strike and then getting a double-play. If there are two outs, that prevention isn't necessary.
As for the reasoning behind the rule itself, I can't find a definitive resource, but speculation is that an out needs a successful defensive player and an unsuccessful offensive player to occur.
The only problem I have with this rule is the ball hitting the ground or plate before it gets to the catcher. Whether they call it a "dropped" third strike or an "uncaught" third strike, it hasn't gotten to the catcher yet.....if he/she catches it when it gets there, it should be an out....similar to a caught foul tip. I see no merit in a batter swinging at a ball bouncing in the dirt being rewarded in all instances.