Maybe when Gandalf warns Aragorn that he cannot harm Sauron through his swordsmanship he is saying more to him than simply telling him it is not advisable. Aragorn is possibly very aware of his own fighting abilities, while Gandalf is keenly aware that Sauron is a very different foe to another Man or an Orc; he is warning Aragorn that sheer swordsmanship is not enough to rid Middle earth of this particular foe, certainly this time around. I would imagine that even if someone were able to get close enough to Sauron to even try to put a blade through him, it might be rather like trying to cut fire.
In addition, it might be that as Narsil was broken, when it was reforged as Anduril it effectively became a different weapon? If it was 'wound about with spells' like the Barrow-blades, would those spells also have been broken when it fell beneath Elendil? To me, it seems that Isildur used Narsil on Sauron after his body had fallen, and that his act was not the act which destroyed Sauron's physical form.
__________________
Gordon's alive!
|