I have to strongly disagree with the argument that the effect on the Surcoat isn't worth using. While I understand the benefits of materia melded gear, you have to weigh how it's going to affect the battle overall.

Take Ifrit or Garuda as an example. Enmity isn't going to be a problem for you as even a PLD can hold solid hate on them. HP and DEF are both important, sure, but if you can maintain your own HP and take a load off the WHM, why not? In doing so, you're constantly adding to your enmity and your mages have a little bit of breathing room.

For a majority of the fights, you have people in range to use Cover, and most bosses hit hard enough to give you excellent returns on MP, thus allowing you to cure yourself (or others) more over the course of a fight. I would certainly take this increased utility over a handful of extra HP or DEF.

Also, since Cover is just a forced hate shift, and you don't have to actually be between the mob and the covered individual to get the benefit of the MP return, it still has usefulness in fights where you use ranged damage. Like against the Darkhold Ogre, you're still going to have someone in range to Cover without putting either of you at risk.