In "The Art of Management" Richard Templer says that you don't need to be able to do the job of the people that you are managing, because it is not your job to do theirs if they are absent. If you do, who is managing? But he does say that you should understand what they do, which might entail learning the basics (but don't let upper management know, or they may expect the above).

This struck me as obvious, but difficult to achieve. Then again balancing these two opposing pressures is part of the managers job.