oopsnearly, I agree with your last. But I wonder if it has anything to do with swapping seats. I encourage my copilots to speak their mind at any point, and they usually do. Even when we've got the fixed-seat policy.
Besides, we alternate PF and PNF duties regularly which also allows for some decision making on the copilots part.
But I suppose one gets used to swapping seats as well as to operating from the same seat all day. I must say that I'm comfortable with the fixed-seat policy.