This has changed somewhat in Stormblood. At this point, melding is a simple game, but it’s more complex than it gets credit for, while also perhaps being a bit simpler than it needs to be. Or perhaps it’s just as complex as it needs to be. It’s a multi-faceted issue, in other words, and one that deserves more nods than it gets.
Heavensward was the current expansion for Final Fantasy XIV, we were introduced to the idea that you could meld materia onto valuable endgame gear. It was a big shift, made only slightly smaller by the fact that pretty much every single job required the exact same melds without the slightest amount of consideration. You didn’t really need to think about it except for a handful of cases, and even in places where melding something else might be useful (like melding just enough Piety for Black Mage to get another cast off), you weren’t going to be suffering if you just ignored it.