Since ilv55 gear can be converted into tier IV materia (can only be melded to ilv70+ [ilv55+ for crafting materia]), I would assume when ilv90 gear with materia slots and tier V materia are introduced, those ilv70 items could be converted into tier V materia. Until then, since the only place to spiritbond ilv70 gear is coil, I would assume we're not really supposed to convert the ilv70 crafted gear (unless you really want tier IV materia and have a group to grind turn 3 with).

My point probably isn't clear, but, what I'm trying to say is that, in the future, the materia that can come from ilv70 crafted gear may be worth it to convert the gear. Also since by that time, 1125 ToP might be obtainable in fewer than 12 dungeon runs, or the prices of some of those materials may be reduced. And if we can relatively easily convert it into worthwhile materia, we wouldn't really have much purpose in breaking it down to materials.

But, that's a ways off, and these are just my thoughts.