Think of it like a debate or congress. Cooperative assumes that they will all follow the rules. Of course this works very well and efficiently if people follow the rules. When the 5 minute timer goes off, the scheduler pulls the power from the Windbag's mic. So it all sounds pretty easy.
Engineering is always about tradeoffs.
Well in theory. Once again that comes crashing into the walls of reality. The problem is that it is those people that decide how important they are. Joe Windbag comes up and decides that he is really, really important, and schedules his priority accordingly, and he gets more time than others. Usually the next person comes along and decides that he is just as important as the last guy, so bumps up his priority, and so on.