I meld materia to low level gear as I'm leveling up alt jobs. Frankly, I do this just because I can with a decent bank-roll and all DoH / DoL jobs leveled. There's no real reason to do so.
Materia melding is absolutely essential for higher level DoH and DoL. DoL is a bit easier but DoH is incredibly demanding for materia. Most of my DoH gear has either 2 or 3 over melds, The jewelry is common but all left hand gear (except the new head piece) plus the off hand are unique to each job therefore must be done 8 times. And the requirements on which materia to use are very specific in order to meet required levels to craft three star items. Since the update, the requirement has changed which has forced me to clean off the materia on my off hand tools and rebuild them with different ones.
Just to redo those off-hand tools will take 32 Craftsman's Command III and 56 Craftman's Competence III assuming nominal success. RNG can be a killer though. The first off-hand I remade I had to use 20 Craftsman's Competence III to finally get a successful meld (the probability of success on that one is supposed to be ~15%). At around 14K gil each, that works out to 280K gil worth of materia for just the one meld.
I even needed to craft new DoL gear WITH MATERIA in order to get the needed gathering for my Miner Retainer to access the highest level Exploration ventures (that one also needed me to quest another new Forager's Dolabra as it was not possible to meet the gathering threshold without it).