Overbooking increases the likelihood of full flights which increases the efficiency of the system and reduces costs. It's hard to see a problem with that.
I think it's a value judgement. Is a lowered price of the majority of instances of a thing (flying or taking a class, in these examples) a worthwhile trade for the reward being a probability rather than a guarantee? Is the severity of the minority case (getting bumped) worth the trade? What actual probabilities for happy vs. sad path are acceptable to you?
Those are subjective questions with no inherently correct answers.
Also there's some worth in having spare capacity. When you're constantly overbooking and suddenly there's a spike in demand, you're SOL and there's no good way out of this.
