It might have offered cheaper drinks than outside in my days both as a student and later as a QFI (it also reduced the drink/drive risk), but in those days the UAS was staffed with regular RAF officers who kept the little beggars in check - and behaviour in the Mess was firmly controlled. Not sure if it's the same now with ancient FTRS and other pseudo-civilians filling many of the QFI posts?
Another sound reason for the UASs is that they can provide good instructional experience for inexperienced QFIs - or rather could do if the slots aren't taken by recycled ex-Wg Cdrs.
Introducing streaming assessment was a stupid thing to do at the UASs as it inevitably led to conflict between academic and RAF progress. When I was a student we were invariably told that our degrees came first and flying second; flying was purely there as light relief to maintain our interest whilst we struggled with the delights of our degree courses.
The highlight of the year was Summer Camp at a 'proper' RAF station - we went to Thorney Island, Marham, Newton (hmm) and Abingdon. And St Mawgan when I was a QFI. But that high-value event which bonded the students into a team is also a thing of the past, sacrificied on the bean counters financial altar....