Was pondering how such a weapon could come about. (The original idea came from a DM giving a paladin a sword, thus widening the possibilities.)
- So no one unworthy could wield the powers. A conceited wizard could do this, or one concerned with hubris.
- So that it does not give people a bad notion of the maker, seeing such power wielded so incompetently. (Even a paladin's sword might do this; you do not want him to bring shame on what he serves.)
- To spur the wielder onward by the prospect of new powers above and beyond his increases in skill, to incite the desire for greatness.
- To protect the wielder by ensuring that his sword always seems exactly appropriate to his abilities and thus he rouses no questions of why so weak a fighter got so strong a weapon, or why so powerful a fighter clings to such a weak one. Particularly appropriate to the paladin if he must operate in evil lands.
- Because the original maker was customizing the sword to fit exactly one person to do exactly one thing, the need for spectacular results in a crisis trumping any need for re-use. The wielder and weapon can slowly mutually adapt to each other to overcome this. Perhaps even there will be tweaks to its abilities.
- A wizard noticed that technique and invented blank blades. As the wielder uses it, the sword takes on appropriate powers. This of course works best as a plot -- if the hero needs to fight the lich king, he must fight the undead even in the lich's kingdom even at the risk of being noticed too soon, and if he needs to fight an ancient red dragon, he needs to fight dragons and fire-based monsters. Merely fighting random monsters gives you a rather generic sword. The wizard gained a reputation as a marvelous sword-maker because every sword he made was the source of twenty legends, and villains gained a tendency to variegate their minions.