[inglorion]

You must not forget the spirits themselves. There necromancable spirits are the unquiet ones, those that have not yet passed away and still haunt a particular place like their place of death. Maybe because they are not finished with life yet or because they do not know they´re dead. Once the spirit has passed away, I think it´s no longer possible for Sauron or anyone to recall them. So the only spirits usable to Sauron are the unquiet ones which choose not to go or cannot go yet. That might also be why the spirits of the fallen in the battle at the end of the 2nd age were chosen by Sauron; because they are still there (Remember the faces of the people Frodo and Sam saw when passing the Dead Marshes).