I'm concerned by the apparent inability to toggle off FnB with the 5.3 version. I see people expressing dissatisfaction with the current implementation, but it's not really all that big of a deal from what I've used it for. Granted, I'm still progressing on 10M Normal, but this is how I see it:
/cast [mod:shift] FnB
/cast Spell
Replace Spell with an ability, and it will spread whatever FnB is capable of effecting. There may be a delay equal to input lag, preventing you from immediately queuing up the next FnB until your current Spell finishes casting, but simplying holding the Shift button as you mash your hotkey of choice seems to work just fine.
In the new design, with the reported lack of deactivation toggle, we're stuck casting FnB-buffed spells until we either CancelAura or run out of embers. I see this as being problematic when we want to stop AoE to conserve embers when the number of targets drops too low. To me, it seems like it will add a hotkey to the mix when we want to switch back to normal spellcasting. I'd rather just press Shift when I want to AoE and leave it off when I don't. I like making a conscious decision to AoE before I press the button, as opposed to having to turn off AoE later. Then, let us not forget the less-skilled (newb) players. Can't stop AoE until you run out of gas? Not player friendly design.
The only saving grace I can see with this would be having the ability to un-FnB a cast in-process, assuming it checks the FnB condition when the spell finishes casting instead of when the cast begins.