Early in my corporate career I worked for a very Dilbert-esque where the managers had an annoying way of buzzing around what they actually meant to say. Even when you figured out what they were getting at, they refused to be pinned down.
Many of those conversations sounded remarkably like this:
37 Pieces of Flair from Lloyd Wilson on Vimeo.
This has always been a problem in teaching. Open-ended questions and instructions are highly useful, even necessary when trying to get students to be more independent and develop problem-solving skills, but the flipside is you have to be prepared for a response other than the one you had in mind.You also have to listen closely and decide whether a response was right, wrong or somewhere in between. Open-ended questions are best used as part of a conversation; they can be highly problematic as part of a written assignment.
The problem has gotten much worse with the rise of the education reform movement. As mentioned before, the movement has a history of picking up techniques but leaving behind essential context. The result has been a steady stream of students given vague questions then penalized for having the "wrong" correct answer.