Most performers "mark" when they're tired during rehearsals. Are they "not giving it their all" or are they getting quite a benefit from doing this? You'd be surprised at how beneficial marking can be.
If you're not familiar with marking, here's a definition from the authors of a recent study on how marking benefits dancers: "Marking involves enacting the sequence of movements with curtailed size and energy by diminishing the size of steps, height of jumps and leaps, and extension of limbs. The dancer often does not leave the floor and may even substitute hand gestures for certain steps."
Psychology in Everyday Life: The Psych Files