Radar 707: Wheelybin has adequately explained the reasons for MDIs. However, your apparent disapproval of this method of managing traffic intrigues me.
If we can use MDIs or speak to neighbouring units and get them to give us radar headings or miles in trail or whatever, and thereby protect the sectors whilst moving the traffic with minimal delay then surely that is a good thing. A flow restriction is actually a very blunt tool, for the reasons Wheelybin has already explained. If we were to put in a flow measure to achieve the same result as is managed with an MDI you would probably find the delays are much larger than a 1/3 or 1/5 MDI will create. The workload for ATC and Airline ops in managing the slots would also increase.
Traffic management is developing, and flow restrictions are only one of the methods. It has nothing to do with fudging it to meet delay destinations, and everything to do with balancing the demands of the airlines with the limitations of the sectors and protecting the controllers from overload.