Duration is the time between when the task starts and when it ends. So if a
task is complete, it has a start date and a finish date, and thus it HAS a
duration. How could you possibly "track a task to complete" if you didn't
know if it started? If it started, how could you know that if you didn't
know when it started? Likewise, how could you track it if you don't know
whether or not it is complete, and if you know it is complete, how could you
not know the completion date? Those two dates define the duration so why is
it a problem to have the duration populated?
A project plan is not just a checklist of items to be done. It is a
schedule of when the items need to start and when they are expected to
finish so that the resources will know when they need to step up to the
plate and get their jobs done.