Another issue to onsider is workload management. Once you crack the brakes open your brain is committing a little bit of processing power to keep your hand in place/watch for pitch changes, etc. This is at the point at which your workload in the circuit is increasing already - lookout, speed control, rate of descent, lookout, rate of turn, undershoot/overshoot? gusting, increased/decreased sink rate, lookout, landing area, other gliders in circuit, where's the tug? is there launching taking place?, lookout, etc. etc. Why would you intentionally want to increase your workload at this point?
Most competent pilots should, by the time they're solo, be able to deal with the increased workload but that doesn't mean they need to. Indeed, adverse conditions aside, I always suggest to pupils that if they've ended up in a situation where they need to use airbrakes in the turn then they've misjudged the circuit already. If they don't have the skills to manage a simple circuit, why should I believe that they have the skills to deal with an increase in workload?