Instructors are taught to teach pilots to turn final, get the glider settled down in a stable approach and not to open the airbrake until they're on the 2/3 airbrake approach path
Then its not a stable approach is it?!
And what happens when you have a cable break at a small site, or an aerotow rope break at a very awkward height? You may well have to use large need large amounts of spoiler/airbrake in a final turn to complete the proceedure safely.
If the pilot cannot be relied upon to use airbrakes in a final turn he is not really ready to fly alone!