I'm not an MCC instructor
But when I did the MCC course we never managed to crash it. However nearer the end as we had finished the course we were allowed to "play" with it.
So in our "wisdom" we attempted a barrel roll in the King air sim just for fun.
The plane crashed!
However the question being asked: "Should you let them crash?"
Well if you deem they should be able to cope with what you just taught them, yes.
Then debrief where it went all wrong and how it could have gone differently.
Again I'm not an MCC instructor. Just an instructor.