While I think what you're proposing is the ideal, the question becomes, what are the tasks that can be reasonably done in e.g. a week? Would the average "good" employee finish them in 20 hours? 30? 40? That's why I think there should be a guideline in the number of hours.
This will add overhead by always keeping everyone on track with specific tasks, but it's definitely worth it (think Scrum/Agile).
Doesn't Best Buy do this?