In the context of Rimworld, and in particular minor breaks, I always figured mental breaks were a way of saying "Pawn has had enough and decides to disobey orders". They do whatever they want, particularly things that they enjoy, to relieve stress. Perhaps the breaks listed don't qualify as breaks in the medical sense, but they're (mostly) things you might see someone doing when they need stress relief, and they would, for the most part, make the game more interesting, not less; that is the essence of game design.