It is entirely possible that the MoD has done a cost/benefit analysis and worked out that the cost of beefing up the flying training system with extra airframes, maintainers, QFIs etc. is more than the cost of keeping guys and gals holding.
Having built up such a capacity it then needs to be kept utilised, or more is wasted. At least having people holding ensures (or should ensure) max utilisation of assets. If student aircrew numbers start to decline, all that happens is that people hold for less.
That said, I can't think of anything worse - in my time RN lookers got 6 months on a fishery protection boat, pilots were going straight through...