No clue what the original intention was. As soon as you hit Level 50, 100% of the Gear (crafted items left aside) you get in dungeons or can exchange for tokens has no slots for any Materia anyways. And even below Level 50, there is no real purpouse. Stack yourself with Earth resistance in Titan HM or Wind Resistance for Garuda HM and you will still die because of the amount of Damage you receive. The reduction of damage is so low that it can't be taken into account. Only purpouse of Elemental Resistance right now is to use them for spirit bonding.