Are you at the planarite cap and don't know what to spend it on? In Sanctum (And Meridian for Defiants) There is an Ascended Powers vendor who is often overlooked.
They sell 3 abilities that you should buy.
Guardian's Flaire (Defiants Flaire for Defiants)
This lets you summon a group of NPCs to help you if you are in trouble.
Summon Rescue Medic
Probably the most usefull one, this ability summons an NPC healer to mend your soul. Your Party/raid can use it too!
This ability lets you see and pick up actifacts that would not otherwise be visible. (e.g. Twisted artifacts, have a red instead of a white glow.)
The other thing to spend spare planarite on is by buying special lures from NPCs such as Anden Frey in Durnheim. For 125k planarite and 200 void stones, you can buy a lure that can make some good money if you can join a raid group to do the event.