I suggest that the main thing a cadet got out of the flying he did in the ATC was a massive boost to his confidence. Flying solo is a character forming experience no matter what your educational standard might have been. I was very confidant young engineer - I had an honours degree in engineering, and had been trained by Rolls-Royce, but the experience of that first solo (and the 2200 others that followed it) did more for my self confidence than anything that had gone before. The potential outcome of not getting that first solo right was uppermost in my mind. That was why I wrote in an earlier post that I thought anyone who had taken part in an activity which might result in death becomes a different sort of person.
What you actually fly seems to me to be quite irrelevant. The 'kick' you would get from a solo in an ASK21 is no different to the one you and I got from a T31. Also no matter how good you might become at flying a modern glider, the RAF will assume quite rightly that you may well have the capability to fly an aeroplane, but you will still need to go through the same training programme as a chap who has had no hands on flying experience.
Which brings me full circle to saying that I think the ATC would have done far better by buying K13's or something similar - KISS. Just my view. A modern plastic glider flies much the same as a T31, or a Tutor, or even a K13, it just does it a bit longer and a lot more expensively.