Earthly star needs some prior knowledge of the fight, and sometimes will not be used on CD because you want to keep it for a specific raid AOE.
For instance i only use it in Suzaku Ex for that share after individual marked players, because most parties groups back in the back of the boss and I know people will be there. But during the colours phase it is a nightmare to optimize haha. On O9S i use it for big elemental attacks (especially the earth one as tanks and heal need full heal), akh morns on O10S...
Maybe I could milk one for free DPS or tank AA healing in between those specific times, but I have yet to look into it.
And it also depends, if you have a WHM you can ruin an Asylum / Medica II with an Earthly Star, and with a SCH you can have your ES rined by an indom too...