It's very very dumb instructing !! The student will soon realise that you don't have confidence in your own abilities, and draw their own conclusions. Highly unprofessional.
If you can't do the exercise yourself under the same conditions you're asking the student to do it, then there's a substantial lack of ability and credibility. Lack of ability should make you unemployed. Lack of credibility inevitably leads students to request a different instructor next time. And rightly so.
DR is good, and it works, and the batteries never go flat; but if the airspace is complex, or you're unfamiliar with the area, or there aren't too many good landmarks, you may be foolishly tempted.
But if an instructor isn't competent to teach any particular phase of flight instruction, then it's up to them to become competent before they attempt to pass themselves off as knowing more about it than their students.