Cardiff are responsible for more airspace than Manchester sub-centre when that first opened. They move a large amount of traffic in a professional and expeditious fashion and they make my job at Swanwick a lot easier. Would you agree that it would be totally ridiculous to impede scheduled traffic in the event that the airspace was saturated with training traffic?
Traffic handling capacity is a finite resource and one method of matching supply and demand is the requirement to obtain prior permission [get a slot].
Reality check....ATC is an expensive business....exactly where does the revenue come from.....does it come from the PA28 clogging up the hold for an hour?.....or from the heavy aluminium tube that pays a landing charge and a couple of hundred passenger taxes and thereby keeps lots of people in work?
The days of the big un-restricted freebies are gone.
By the way....does the sched driver mind being vectored out of his/her way [with the associated fuel burn penalty] just to fly round some spam-can in the hold?