I expect that most people will not bother to do much melding, to be honest. Few bother to meld their crafted gear from 50 to 60, if they even get any at all! Oh, at first, yes, there will be a mad dash to the Marketboard for Materia. Afterwards? I doubt it.

The big question in my mind is whether those primary stats will be at their cap already. Will the Monk Boots, for example, even take a STR Materia? Or a VIT Materia? The very idea of being able to meld primary stats to the open slot, but not to any hidden slots, implies yes; but very few crafted HQ pieces (although not crafted, the dungeon/tome gear currently counts as HQ) take any more of their primary stats. I don't want to be forced to put DEX Materia into my monk boots, or STR Materia into my bard's gloves, because the relevant primary stats are already capped ...