It's a disincentive for ignorance, but not a good one. Nobody should ever be shamed for not knowing something, even if you personally feel they should know it, so long as they are willing and eager to learn it. If they stubbornly refuse to learn something after it becomes clear that they'd better for knowing it, thats when it's ok to feign surprise. But for goodness sake at least give them a chance to learn before you shame them.