It sounds like you had the quality side taken care of by another means (rubrics) which is admirable but what I'm saying is that basing rank on the number of posts doesn't itself promote quality. Many less-thoughful teachers or those new to online learning would come across the feature and assume it is "the way to do things".
One place Moodle differs from generic forum products is that it always tries to promote good online pedagogy.
In most online classrooms you really need to encourage reflection, connection and thus quality posting, because quantity can rapidly scale out of control (eg 30 students x 4 posts a day = 120 posts for everyone to read every day).
In Don's case though (language practice) I can see how cumulative scoring could be a good incentive, so perhaps there is room for both methods after all.