I’m not sure I agree with you about the valuation of ancillary functions in open source projects. Documenting is certainly seen as less prestigious than coding, but it’s certainly appreciated, especially by the people who read it.
Project management gets shrugged off not so much because it’s seen as unnecessary, but rather because unlike documentation or artwork it’s pretty easy to pretend to do the job when you’re really not qualified. Further, unless you’re contributing on your employers time, you’re really only going to be interested in scratching your own itches. If a community project manager tells you to work on x instead of y, you really have no reason to listen to them. Even on the planning side, you can only wield the carrot or stick of getting into a particular release.
The successful community project managers aren’t just that, they’re also lead developers who have the justification to say “This is where we’re going, this is what needs to be done, and this is when it needs to be done by. Feel free to pitch in.”