Considering not all students are interested in every topic of a subject or whole subjects, I like the idea of extrinsic motivation because as per the definition, it provides students with something positive to look forward to.
However, in the adult classroom, I think in terms of praise or materialistic rewards, it should be rare and given in public because of several reasons. One, when the adult student is finally rewarded with praise after many attempts, it holds more value because it was harder to achieve. Not to say, in a semester's worth of classes, the instructor should be so… >>>