I personally think they are unnecessary and complicated. We worked on using them and I believe they take away from the instincts of the game. I elected to forego the cards, I thought they Footballized the game too much. A good coach or player can steal signs from tendencies, they don't need to hear or see a sign. Many young pitchers will give away what they're throwing without knowing it. Catchers will often setup early and at the same location on certain pitches.
A team can always get a hitter to show bunt by stepping off or throwing to 1b.
When dealing with youth ball and high school, I think the play on the field can reveal a teams plans as well as a young player who doesn't hide the upcoming scenario.
I'm sure there are good arguments on both sides, but I don't think pro ball will ever consider them. And in that case, will college players become too used to them and then have to be retrained when they move to pro ball?