The Incubus King items are special "signature" rewards. The way Sierra Lee structured the game, there's exactly one of these key items per chapter - with the Incubus Pin from the Ancient Ruins being the collectible that belongs to chapter four.
This means that you have time up to and including the free roaming section after the Erosian War to collect this item, before the chapter concludes. Once you transition to the next chapter, you could still open the Ancient Ruins, but the Incubus Pin will no longer be available.
We don't know the future effect yet, but I'm willing to bet that collecting the entire set of IK items will make a significant difference. And yes, it's definitely a tradeoff - between being able to afford other investments, and spending the ProN to open the ruins in time for aquiring the pin.