Luffy won't eat the fruit because it would go against this statement from Oda in Volume 66's SBS:
Um…couldn't Luffy still strength, just while on fire, if he were to get the fruit? I don't feel this statement is what invalidates this idea. Any other statement, I dunno. This one just has a few obvious flaws.
Getting a fire logia wouldn't even give Luffy an advantage, at least against the guys he doesn't already have an advantage against. Not at this point of the story, where a ton of dudes can belt a logia and break Luffy's rubber defense with the same technique. Blackbeard has Whitebeard's ability, and he wiped the floor with Ace on a daily basis. Akainu beat both Luffy and Ace in their last encounter. If Luffy were to gain Ace's DF somehow, perhaps after he beats any enemy you can think of that can't beat Ace, he'd still struggle against the top powers of the world.