The more people that understand the process, and what goes into it, the better.
This is actually why the PMP was originally created. There's a reason it's called Project Management Professional, and not Professional Project Manager. ;)
In the beginning, the PMP was designed to certify that team members, not the PM, had a good understanding of the general processes of project management. The thinking was that the more the team knew about what went into managing projects, the more assistance they could provide. Knowing the how's and why's of what the PM is doing or why they want certain information helps the team members understand better, and possibly provide more accurate or relevant info.
As Mark said, this doesn't mean that the PM should be overruled or argued with, as they're ultimately responsible, but it helps if everyone's speaking the same language.