Crystals also made a cameo in Super Mario RPG (I think they used Final Fantasy music for the a fight too), as it was in an old Final Fantasy anime that I've noticed a lot of people don't know exist. It's not really recycled anymore than Chocobo, Biggs & Wedge (Even a Chrono Trigger cameo), Cid or Moogles are recycled...it use to just be the thing, and eventually it was used less and less in current stories or they are just called something different. From my understanding the crystals are returning in FFXV.

If one had the time I'm sure they could create an ultimate timeline to connect all the games through the crystals, Materia, Espers or whatever.