You are right Behl!
Annona branches pruned off and thrown on the ground in the winter will bud out in the spring--- so your having grafted some of it can end up being entirely irrelevant.
All the reasons given by Adam are also valid.
There is another reason often overlooked--- expectations of magic --- if the roostock doesn't have a big fat seed full of stored up food, and you go and cut all the leaves off the roostock, what is the poor thing going to live off. [And remember, fertilizer is not "plant food", it is just nutritional mineral supplements, none of which are an edible source of energy (carbohydrates, lipids, or proteins.] Roots starve first, or get too weak to fend off rots.