A task will take the time that have been reserved for it. That is inefficient. Changing to a result-orientated approach where an employee is giving weekly or daily tasks (if it isn't too complicated - which is really a good question) would also mean that if an employee is finished with his weekly tasks after 3 days, he gets to go home. For many seems counterintuitive (fixed time management is essentially used from the first day of school and is pervasive in society) and it does have it own difficulties changing. For instance, how does co-workers feel when one fast employee always gets to leave early? How easy is it to pick tasks for employees?
Freelance programmers are one group where this result-orientated approach works, not sure it can be extended to other sectors easily.