a friend of mine used to use a pinky hole in almost of all of his equipment (think he stopped tho) but he said it helped him get around the ball better (more side roll, axis rotation)
yes its legal in pba and usbc comp, make sure statics are right (usually have to put a weighthole on other side of ball (in track)
and i no someone else that doesnt use there pinky hole all the time, they take it out for drier conditions etc