I find it useful for dressage, to help the rider to get 'deep' into the saddle as well as ensuring that legs are long and stretched, which I find helps me to follow the horses movement
In the very novice or unbalanced rider I feel its generally counterproductive as all it does is cause tension and pain for both rider and horse. For those who need/want to refine their position then yes it is very beneficial.
I fail to understand why many BHS type establishments insist on beginers/ novices being subjected to it as part of their training. Either teach a rider right from day one to sit correctly with no stirrups and then allow them once core stability and balance are established or wait until they have enough strength to begin to cope with the change from stirrups to no stirrups in the lesson.