So I've been playing a BS/FA from before the FA buffs (she was my first character, i didn't know FA was bad!) and here are a couple thoughts:
-With reasonable IO investment, you can easily get to melee softcap with just one stack of DA. Before then you can double stack it to get the same thing. It does make an otherwise very squishy secondary more survivable.
-Fiery Aura + Build Up gives you a TON of burst damage. When something absolutely must die, you can kill it really fast.
-BS/Katana are more single-target sets, Burn gives you a lot more AoE. I took Fireball from the APP; Whirling Sword was just too slow for me.
-While leveling, you should go to Crotoa and laugh at the Fir Bolgs as they attempt to harm you.
-Fun fact: because of its +resist component, Healing Flames actually works on Hami raids.
IMO, instead of having synergy, it's more like the sets cover each other's holes.
It also seems the game designers are beginning to indirectly nerf pure +Def by giving the new content lots of +ToHit, which makes resistance-based sets (and the big self-heal) attractive.
