[magikbum]

Opposing player on Second base could see the sign from the catcher and then sign his own back to dugout or player.

Player batting can see his own dugout or his own third base/first base manager and easily receive a signal from them.

As is now all major broadcasts easily show the catcher's sign and it can easily be decoded back to his pitches. Without the use of cheating you could only really get the signs today by having a player of your own on second base.

[thefalcon]

Which is why signs are obfuscated whenever a runner is on second base.