Chima is introducing three new weapons: a new glowy sci-fi axe head and two new glowy sci-fi swords, one good and one bad.
The running theme of these parts is that they fill niches which can't be filled by other parts. As important as Aragorn's sword is, if I recall it's a fairly standard shape, like the sword currently used for him in sets. Sting, on the other hand, is a fairly unique shape, which would be ill-depicted by existing swords and knives, thus necessitating a new mold. It's less about how important they are and more about whether existing parts are sufficient. Consider that almost every Jedi and Sith in Star Wars have lightsabers with unique hilts, but most lightsabers in Lego Star Wars use the exact same hilt, except ones which clearly have a different overall shape like Count Dooku's.
Actually the old chromed Kings sword would probably work well for Anduril. It's longer than the new CMF longsword that Aragorn currently has and stands out as something important. The only problem with it is the rounded tip..