1) Should hubwheels be added to PMs?
Why would you? Is there a reason to other than karma whoring? (Serious question, not trolling.)
2) Should the OP be able to add more people to the PM? This is currently possible.
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?