The whole league wasn't doing it and it did have a meaningful effect Knowing what pitch is coming is enormously beneficial to major league hitters. That's why you see otherwise effective pitchers getting their brains beaten in if they start tipping their pitches.
It didn’t have a meaningful effect. By the time the sign is being relayed to the batter the pitch is already coming. There isn’t enough time between the catcher giving the signal and the pitcher throwing the pitch. Statistical analysis didn’t show any benefit.
The Red Sox got caught doing something similar which Pedroia was involved in. He explained to a reporter that there isn’t enough time to pull it off. When asked why even bother, he said for the challenge of it and for fun.
I say the whole league was doing it because ever since they allowed instant replay, pitchers and catchers began getting their signals crossed much more often. That’s because they began changing signals during a game more often. Catchers would then have to head out to the mound to clarify the new signals. Remy and Eckersley once explained what was going with home teams stealing signals from the replay booth before the Astros were caught.
There are many players changing teams between seasons and during/throughout each season. Any player to a new team is going to tell that new team what his previous team was up to. There are no secrets. Plus, there’s only so many ways to relay this to the batter. The other team picks up on it. The whole league new.