Apart from the fact that mildly aggressive questionning doesn't help the debate, wouldn't the cost of paying for an extra shift, one of these "X" staff, normally be a lot less than the cot of cancelling the train?
It would cost a lot more than getting a driver in on a rest day when its actually needed rather than have drivers sat around being paid to do nothing 'just in case' they ae needed.
At our place we have one morning cover turn and one afternoon cover turn and that's it for a depot of 100 drivers, sometimes these are left uncovered if things are a bit tight so we end up with no cover or spare drivers at all, saves a few quid for the TOC and if a train is cancelled they just blame it on a shortage of drivers (without going into any details as to why), and the great british public naturally assume its the greedy overpaid drivers fault rather than the TOC who are running things as close to minimum as possible.
Oh and I have had people moaning at me because their train has been cancelled due to no driver, I just tell them why it is there wasn't a driver but I don't think any of them actually care as its just easier to blame the drivers.