It is hard to justify this recommendation until you consider the possibilities for an alternative. If you hold honest admission of ignorance in high esteem, you may direct students to refrain from rephrasing the question in their own words and embellishing upon this with inappropriate but related verbiage. However, considering human nature, it seems likely that at least a few students may not adopt your philosophy or your advice and try to fake it in the hope of duping you into giving them partial credit. Though this strategy will tend to minimize the influence of personality on scores, it will probably make your task of grading more difficult and will certainly increase the amount you have to read. For these reasons it is probably best to be vague.
Unstructured questions are to be preferred over questions focusing on a specific problem.
The ability to demonstrate creativity and original thought can be demonstrated most effectively by having the students respond to broad topics. Thus a question which asks the examinees to describe the solar system would provide greater opportunity for scholarly writing than would a narrowly focused question which might ask the examinees to explain how Bode's law helped lead to the discovery of asteroids. Moreover, broad questions allow you to test an entire unit of instruction with only one or two questions, thus effecting a substantial saving of time and also forestalling the often heard criticism that the test did not cover what the students studied. Offer a choice of questions on which to write. With each student answering a different set of questions, the tedium of grading is reduced considerably.