A possible compromise might be to have the PM functionality officially available (but as a plugin, not as part of the core). It would be written (or at least vetted) by bbPress staff… and even downloaded as part of the standard distribution. But each user could choose to enable or disable it, and it wouldn’t be in the core.
I think we’re getting close to that actually. BuddyPress has a PM plugin on its roadmap… maybe that can be adapted for bbPress.