I was in Army Cadets and I was an armoured gunner in the reserves. It doesn't help all that much, except maybe the drill. Cadets can do little to expose you to the military way of flying, working in a multi crew aircraft, and completing missions. I was never in Air Cadets and I made it through, I don't think the road would have been any easier if I did Air Cadets instead of Army Cadets.
It's not a waste of time, it does give you some perspective on how people react in different situations, etc. There is no such thing as bad experience, it's all experience and it all shapes who you become as an adult. So my advice, do as many courses and cool things as you can do and enjoy it while it lasts.
Cheers