Its a pain in the butt when attached to the back stay.
When you go to drop your sail, somebody has to grab the end of the boom as it falls and than attach it using the snap hook. Or you have to be dead nose to the wind and attach the boom to the snap before you lower the sail. We had this arrangement many many years ago. I remember more than once, we would attached the boom to the back stay, the person at the helm would loose concentration, fall off the wind just a hair, and the sail would fill. Than your stuck because you have no way to release the boom; its attached to the back stay. The helmsman must than try and get her pointed into the wind again to get everything back under control.
Much simpilier to have it attached to the top of the mast as normal.