I figure the "real use" of the pendant is purely for worldbuilding, and in fact is explained by Manus' own Broken Pendant. While the Broken Pendant serves a mechanical purpose, it's described as having some secret power that can't be tapped by the people of the presenr, apparently derived purely from "nostalgia." To Manus, the pendant is all important, though it would appear his power isn't derived from the pendant itself, but rather it awakens his own latent potential. When it was stolen away from him, his power was unleashed in a fit of rage. After all, it was all he had, and didn't hold any true value for civilization as a whole, so why should anyone take it from him?
"Sentimentality" and "nostalgia" go hand-in-hand, so if anything, the pendant could merely be a purposeless treasure that the Undead who carry it feel the desire to protect with their entire being. It's a very *human* reaction, to want to protect simple trinkets of little worth, if only to remind us of happier memories. That certainly ties in with Dark Souls' characterization of humanity and darkness as "foolish."