With a guy like Chapman who has been around the league long enough, guys know what his slider looks like and does. Again, this isn't like Cole Hamels is dropping a slider on them (which would be a shock, since he has never thrown one). Yes, the pitch itself may vary from game to game a little bit, but it can also vary from inning to inning. Or even from pitch to pitch within an at bat. But generally, you know at this point, how much the pitch is going to break. You know at this point how fast the pitch is going to be too.
Yes, if you have never seen him before in your life and you didn't watch any video or read a scouting report on him, seeing him a bunch would make a real difference. But for most Major Leaguers, I just don't buy that seeing him more is going to give them some advantage on hitting any given pitch unless they are: A) tipping the pitch, B) losing velocity/movement on the pitch because of wearing down over the game/year.
If the pitcher is using the same mechanics/velocity to throw the pitch each time, the batter needs to know that the pitch could be a possibility, be able to read that pitch versus other pitches, and know the strikezone. None of those things are skillsets that change by seeing a pitcher more.