Well... every resource is technically infinite in this game for it's lifetime if you argue it that way, it really depends on what you need from the soul points if you've already summoned D'Art and Mordred. If you haven't, advents are a hella awesome way to gather quick soul points so you can summon up that legendary soul you so require. Besides, I'm sure you'll stack up enough of the N and R level materials that using some on soul points after wasting them all on half-elixirs really doesn't hurt, provided you're plenty capable of gathering the eidolon and SSR weapons.