In addition to allowing the export of overall grades, offering an option to export the rubric criteria points for an assignment would offer instructors more insight into the performance of the class.
For example, if the rubric contained a quality of writing category, downloading the results would allow instructors to easily identify who received low scores to refer them to tutors.
For those courses with multiple instructors, it would provide a class-wide review to see if an instructor was grading certain criteria differently than others and be used to drive consistency.
It could be used to identify other trends and possibly assist with curved scores.
This might be best suited to individual assignments rather than tied to exporting the entire gradebook