So far I've attached materia only when story and quests required it to achieve higher level crafts easy.

Beyond that I don't know why I leveld these DoH jobs to 50 to attach materia because (and correct me if I'm wrong) no gear past ilvl50 has any slots in them.

The stats on materia look very cool but who actually uses these?

Spending that much gil on mid-level gear is a total waste of money no matter how you look at it.

So is materia useful for anything else than getting the melding achievement? If so then how do you use the system?

Clearly I'm missing something here because I can't understand what this (admittedly really cool) system is used for.