While I sympathise with the idea that civvies are more expensive than forces - its actually the other way round.
If you play around with DASA statistics, you'll find out that about 70% of the entire MOD civil services (i.e. pretty much everyone up to, and including junior bands of Grade C2 - which is mid level management) earns £26K per year or under.
By contrast, pretty much everyone in the armed forces over the rank of Cpl earns £26K per year or more (which is roughly 80%). This figure does not include the not insignificant allowances package available across the rank spectrum.
So putting a junior CS on the duty watch immediately generates significant savings and frees up a trained sailor to go and do something more useful and valuable than sit in the gatehouse. Thats why the MPGS exists, in order to get useful manpower off the gate and either getting down time or doing more useful work.
I would also point out that the Levene review noted that the Forces are often misemployed in jobs which don't require the levels of skills, training and cost that they require, and actually recommended an enlargement to the size of the MOD CS to put much cheaper CS into jobs to free up forces manpower to do more useful work.
I am afraid that I do not consider the 'duty watch' at COLLINGWOOD to be a particularly challenging task (and I've done my fair share of OOD duties in rig at major shore bases). If the MOD can save money, keep sailors off gash duties and free up bodies to ease pressure elsewhere, like in HERRICK, then thats a good thing in my book.