I totally agree about what you said. But the benefits would just be minimal. It wouldn't be over powering the other equipment at all except in certain areas where it matters. Raid equipment would still be far better overall in most situations due to higher ilvl.

If you used a DF then you would equip your standard gear. If you're a tank going into Ifrit Ex you go ahead and slap in your fire gear for the extra defense. If you're in a perform party you can call out of anyone has any other gear and synergize with them.
It wouldn't be a mandatory system. It would help, but not overtake. And it would greatly benefit crafter's and their gear. An optional system to help if you got it. That was my mind set when thinking this up.