Ethically speaking, I do not believe any project manager should feel the need to work those under his wing to exhaustion. The overall goal and objectives for the project are set forth in the beginning and if you are working your employees to the brink of exhaustion then you as a project manager did not plan accordingly or ask for sufficient resources. As a project manager, he or she should have the best interest of those working on the project as well. If the team is able to achieve the original goals set forth, there should not be a pressing need to push the team to exhaustion to achieve other milestones that would be a nice to have. There are other ways in optimizing a process in order to achieve extra milestones within a project aside from the original goals such as delegating resources effectively.