If its a well executed pin, I'm going to feel that before I try to roll out. I've seen young, flexible folks roll out of pins most of us couldn't and it is a wonderful reality check for those doing the pinning....
Well yeah, frankly if the pin is well executed you are off ballance from the cut until the mat. There is no time, nor is there any footing to get in any real position to flip out.
I've seen people try to flip out anyways..and they jam their shoulder.
I would like to say that it is typically a new student that would try to flip out of a pin, mostly because you train slowly with new students to help them learn ukemi etc. Sometimes a new student might take advantage of the slowed down pace, and I've seen them try to flip out of it...again, if I ever come upon this I think I'd let go rather than risk injury.