The significance of Luminaries now is that they are the "Hard" item to get. Whether they will serve any other purpose in the future, or just become outdated relics of 1.0 is yet to be seen.
But, you can be sure that the level cap will raise, and when it does, there will be items better than both. As of now, you need that level of an item to complete some of the crafts out there, whether that was the right decision (and to allow for people to obtain the item that easily) is really more of an academic debate.
If you get HQ on all of your class specific lvl 50 gear, slot in control and craftsmanship materia to taste, you will be able to craft 2* recipes without needing the luminaries. But the luminaries, (and food) help if you are missing materia or do not have hq.