As they are now, rubrics can only go as low as -1. By allowing a lower value, such as -10, -50, and so on, professors could adjust grades for individual assignments. An example being that if a rubric has a criterion of Late Point Deduction, he/she could choose to give 0 points for not being late, -10 for a week, and -50 for any amount later than that. The student's score would immediately be adjusted accordingly. This avoids trying to help professors make sense of the late submissions option in the grade book.
See attached image to view current rubric options.