Throwing in a half pence from the student perspective:
I've had instructors that instruct the way Crab describes, and the way HC describes. I greatly prefer the way HC describes.
And I really despise instructors who spend time in the air explaining theory. I've got the theory, or should have, on the ground. In the air we need to make theory come out of fingers and toes. IMHO there are better, more direct, more physical ways to do that. Many times it's simply a matter of more rep's until those fingers and toes get calibrated.
That said, with respect to the "unannounced" factor, what I don't mind, in fact what I appreciate, are instructors who take basic maneuvers and start to add small challenges. For instance, they tell you to enter a straight in auto on runway X and, when you are established in your glide, tell you to put it on the other runway, etc. Little frills that up the ante, are based in real world possibilities, but that are not so far removed from current skill levels or the instructional plan for that flight as to completely surprise either student or instructor.