This is another case of bureaucratic nonsense which does little to promote a "safe and expeditious" flow of air traffic.
You actually have to go to PMI and witness the way they administer the "system". I don't think anyone has any complaints about the way UK ATC work the system and most, if not all, pilots understand that the discretionary times are for ATC use, etc.
However I feel that until we have a slot related accident (somebody busing a gut to make it or "fatigue") then I am afraid that "slotmania" will remain with us forever. Yes, I am sure there is a need for some sort of flow control but I suspect that in certain cases empires have been built around the system
------------------