Combat class overmelds are purely luxury and world first raiding. They are literally only good for the first month or so of a raid tier, then you replace your crafted gear with Tome and Raid pieces. So 3 small windows within an entire expansion. Even then, overmelds are far from necessary for raiding in the first month, P1 and P2s have very forgiving enrage timers, if you're wiping to enrage it's because people are dying or not playing properly.

For players who don't raid or don't push week 1 P4s, overmelds existing is a source of income. Grade 9's were great profit at the start of the patch, giving the average player easy gil from spiritbond, hunt trains, 89+ dungeons and such.

It's also one of the few things that rewards crafters. If you have maxed crafters and lots of materia, you can make a full pentamelded set day 1. It's a nice little bonus. It's casual players being jealous of crafters having small bonuses for effort that got crafting into the diluted and low profit state it's in, in the first place.