I agree with Tim's conclusion but not about PMs. In my group at Apple, the PMs had no influence on feature selection. Their job was to get engineering estimates and monitor progress. The new features came from engineers and designers, and filtered up the chain to division directors who then usually approved or disapproved them. My feeling is that part the problem is a kind of organizational inertia: what do you do with all your engineers when your builds are now all done prior to release, and the only bugs that will be worked on are ones that would stop shipment of the pending release? The answer was to get us all to propose new features for the next release. So everyone gets caught in this propose/design/release cycle that just has no end.