One of the biggest complaints from our PMs is that there is no easy way to ensure that users stop logging time to a task after it is "Completed". We will have situations were at the end of the month a person it needing to update their timesheet and they are entering hours beyond the date the task was closed. So it becomes a 'dumping ground' for hours. We have created exception reports to help the PMs monitor when someone is logging time to a Task beyond the Actual Completion Date, however, it's just extra overhead for them that takes time away from more important PM functions. They would really like it if they have an way to stop this from happening; they already have enough reports to monitor.
There is one option that they can attempt to stop this, but it requires that they update the Sharing Option for each person assigned to the Task to un-check the "log time" option, which is also very time consuming depending on the number of Tasks that need to be updated and the number of people assigned. So this isn't perceived by them as a "easy" solution for obvious reasons.