I only have 4 units with enhancements anytime soon that I use on a regular basis: Cecil, Refia, Gilgamesh, and potentially Dark Cecil (after upgrades).
Cecil and Refia are both likely to be benched after Tilith and WoL get released, so I probably won't use rare enhancement materials on them, which only leaves me Gilgamesh and Dark Cecil.
I'm not going to bother enhancing Gilgamesh's Snowpetal, because I almost never use him as a finisher, but I do want to upgrade his Tri-Attack (power purecryst). Upgrading his weapon passive would be nice (tech purecrysts), but that's low priority for me.
Dark Cecil's going to be the big investment for me. You basically have to upgrade his passive to give him the full regen each round if you want to use him (support purecryst). His super nuke + 100% imperil costs a total of 4 power purecrysts, which is going to be very expensive, but hopefully worth it. Good news is upgrading his AoE imperil nuke is super cheap, and only needs T1-3 stones and none from T4-5, so I can use that in the meantime while working on his souleater upgrade.
That's assuming of course that Dark Cecil even gets his enhancements anytime soon. There's always the chance that I pull someone with better purecryst value between now and then.
Very longterm, I'll want to upgrade Agrias and WoL with enhancements too, but that's probably not until 2018 anyway.
You can see who needs what enhancement materials once they release here:
enhancements - FFBraveExvius with the typical things may change for Global disclaimer