Like other posters, I am interested in the unintended consequences of this partial step toward automation.
Suppose the conventional wisdom becomes to give certain catchers more of a free rein, batters much less, and pitchers zero. What's the effect on team dynamics or player strategy or the mental aspects of the game? Here's a couple I thought of.
Pitchers have traditionally had catchers they liked better than others, due to framing and blocking and so forth. Will ABS challenge become predominant among these qualities? Will resentment become overt? "You didn't challenge a single one of my walks the last two games, You challenged three pitches just yesterday for our ace. WTF man?" A pitcher can criticize a catcher's ability to block a ball in the dirt without making it quite so personal as to accuse him of doing better for another pitcher. A player will be a man about it, until things come to a head for some reason, and then it spills out.
Suppose the manager tells a batter, let's call him Mattner, not to challenge any pitches unless he, the manager, has told him when he leaves the on-deck circle that the game situation is important enough to consider doing it. Will this bring harmful pressure exactly when maybe you want Mattner to just relax and pick a pitch to drive? Other side of the coin, will Mattner come to view two-out nobody-on situations, where he's not given the authorization to challenge pitches, as unimportant, and develop bad habits?
Someone else brought this up and I find it intriguing: if a batter challenges a strike call and it's upheld, does it give the opposing team (and maybe the "book" on the batter around the majors) important insight as to what pitches or locations the batter has the most trouble judging?
Someone also brought up that "decision trees" and so forth will be above the pay grade for certain players. I wasn't joking when I invoked Yogi Berra's chestnut about thinking and hitting. Some guys, you can just tell, will get frozen with indecision if given too many strategic things to think about. You go up there to the plate with a plan, and adapt as the at-bat unfolds, and that's about all you can ask from some hitters as they cope with 96 MPH balls potentially buzzing their foreheads and breaking pitches destined for the dirt.
What happens after a batter's challenge is upheld? Do batters given a reprieve from a called strike-three go on to bat statistically better than average? It could be that the occasional home run makes up for a certain number of misguided challenges.
I know I'm kind of overstating the direr consequences, but it's to illustrate what I see as new challenges (no pun intended) for the manager to deal with for clubhouse chemistry. That's a key skill that any manager surely has, so maybe he'll know when "a word" will be sufficient and that will be it. Shelton not having lowered the boom yet on Mattner Wallner seems to be an example of intentionally overlooking a minor problem in the interest of something larger.