letting a horse buck as long as it wants does not teach it.!!!
sure it does, as long as you stay on. Bucking is (usually, and i've assumed in this case too) an attempt by the horse to get rid of the rider, so as long as the rider doesn't fall because of the bucking the horse will be less likely to do it again or as severely again. Simple pressure/release, only this time the horse is applying the pressure to the human.
Doing the cirlces or 1 rein stop or whatever are just ways of preventing the horse from bucking so hard that it gets rid of the person. It doesnt teach the horse to resort to things other than bucking in future. What does that is showing the horse that bucking is unsuccessful for the reason it started bucking.