As I understand it, MDI's are a popular way of limiting sector traffic because they do not show in the overall delay figures and thus improve the NATS attributable delay figures. In their defence MDI's are probably the only way of being sure that there is no overdelivery of traffic. Unfortunately on the down side they are a b*****d for the airfields and do little to remove the bunching which we need to control if we are to move more traffic. This is because however we restrict traffic getting airborne, tactical routings for the TMA task will inevitably change the route profile and hence traffic will naturally bunch. One of the problems for en-route is the relatively short sector lengths between the LTMA airfields and AC sector entry. This means late notification of traffic entering the sector and hence a very late realisation that an overdelivery is on its way. Pre NERC there was departure TV and pending strip production which did allow a certain amount of warning. Without these tools it is probably inevitable that MDI's will be used more simply to protect the en-route sectors.
Hope this makes sense
Z