I believe that only level 30 nodes have this trait, and it doesn't take many levels before your natural elemental resistances will be high enough that no materia is required to reach the target. Melding elemental resistance is pretty pointless - though it IS cheap and does help you spiritbond your gear more quickly so, hey, if you're at a lower level and need just one or two more resistance to get double yield, plug in a materia or two and enjoy it for the short time it's useful.