Although rubrics cannot substitute completely for individualized commentary in the margins and at the end of papers, they can save enough time to allow teachers who might not otherwise assign writing to do so. And simply by writing more, students do improve over time. (Like any other physical and cognitive skill, practice with writing does improve performance over time.) So if a rubric will allow you to have students write, then by all means use a rubric.
You'll find, however, that a carefully designed rubric will give you much more payback for time invested than a rubric that isn't as detailed or as clearly articulated. The following sections define the key components of rubrics and collect advice from various sources about the best ways to develop effective rubrics.