Fundamentally, any scoring system serves the useful purpose of ranking the forks of a bifurcated discussion. Reddit without ranking becomes SomethingAwful, for example. It has little to no effect in a small discussion, but in a large one they have outsized effects; try browsing a single thread on Reddit using different ranking, for example. I can see scoring being useful on a large discussion held via PM... but again, I see a big murky set of possibilities when PMs become things with ranking.
Ok, I can see that being useful in a PM chain with multiple people. I still say that the hub wheel should only be on the public face of the site for the exact reason you have stated elsewhere: it creates an incentive for a private "subreddit."Fundamentally, any scoring system serves the useful purpose of ranking the forks of a bifurcated discussion