Well, I call it a gimmick because it's not something I'd give up ~3k Def for. Having a balanced plate set offers a lot of room for learning bosses, offsetting lag, and carrying you through a bad day of playing. Att sets are really if you feel confident in your playing, which usually means soloing (where the boss is predictable and lag is nonexistant). It has it's uses of course, as I still get 7.5k Def in my WB/F set, but I want to repeat a point from my first post.
By the time you'll be able to be effective with an Att set, you should have ample AP and experience with the game to know that it's viable for you.
If you're at the in-game and skill level which you could perform well in an Att set, chances are you've played your character extensively and are probably already done ranking most core skills. At that point, you'll know that you want to try it for yourself and my input isn't relevant anymore.