We have one old timer instructor in my club who insists on airbrakes fully in during turns. The only good reason I can think of for doing this is if you have a student who is in the early stages of learning the circuit and landing. Early on they may have trouble reliably determining whether they are above or below glidepath while turning or may be overwhelmed by adding the modulation of the brakes to the demands on their attention during the base and final turns.

If you are flying the pattern at the approach speed recommended by the manual for an approach in which full airbrakes might be deployed (which in my opinion is every approach) you should have a safe margin above the stall speed in the sort of well banked turns you should be using in the circuit whether the brakes are in or out. The brakes are there to put you on your desired glide path. Use them as necessary, you paid for them!

As an experiment try this at altitude: trim to the recommended approach speed with full airbrakes or recommended approach speed found in the AFM (or if it's not specified use the formula you were taught for choosing an approach speed), roll the glider into a good coordinated 40 degree bank turn then move the elevator progressively back. In every glider I've flown the elevator hits the stop without provoking a stall. I tried this after reading a Derek Piggott article suggesting it.