The additional fire damage won't be resisted by the bosses, otherwise there would be a bunch of specs that would be nerfed right off the bat. You are getting extra fire damage because it will fit the theme. You walk in, punch an elemental in the face, fashion a suit of armor from the carcass, and use its power to light other shit on fire.
Cmon blizz look at the Tier 12 bonus for warlocks and think about this really hard.
I mean it's already like those in the game( The imp from 'Bane of Doom' and the SB/Incinerate is the 'Shadow and Flame' talant in the destro spec!!!!)