Why would you? Is there a reason to other than karma whoring? (Serious question, not trolling.) I see a PM as an email, a way to resolve a dispute or ask for more info privately. PM's are where you go to ask for clarifications, try to find intent and get a more precise response that you can't, or won't, get in public. The question seems to be, IMO, is Do you want to create a non-public component to Hubski? 1) Should hubwheels be added to PMs?
2) Should the OP be able to add more people to the PM? This is currently possible.
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