Last year, I made this call (kept hit batter in box) four times in a span of around 100 games. That was a little more than usual, and I would say that in the past I might call it twice a season. One was in girl's fastpitch, one was men's fastpitch and two were in high school baseball.
Only one of them was on a batter that stood like a statue in the box and got hit by a really slow pitch. That one dropped down and got the batter in the foot. Absolutely no effort to move was made at all. That was in the girl's fastpitch game.
The other three were a little different- batters who made some token movement away from the pitch, but purposely (in my opinion!) stuck an elbow out to allow the pitch to hit them. Surprisingly, I didn't get any argument on any of those- just kind of a sheepish look from the batter, sort of like, "You caught me!".