@GKaiser:
Nonsense, every logia in the manga (:San Ace, Croc, and so far Caribou) are users of Haki. Whixh they actively used throughout the war.
If Ace knew how to use it, he would have been shown to use it.
The only time the Admirals used haki was (assumingly) when they stopped Whitebeard's attack from destroying the scaffold. Every other attack involved them spamming their logia attacks. And Smoker doesn't use haki.
I also added one more thing about that not being a fight so much as it was a one on one confrontation.
A person would try to see if his powers are effective against somebody because 99.9% of the time, fire would either burn the opponent or stop the opponent like with Aokiji earlier in the war. It makes sense to try to see if a person's ability to burn people would be effective against somebody else. Akainu's power wasn't. But by the time Ace realized that Akainu was attacking Luffy already.
@Coruscation:
But I agree with your overall message. Ace clearly had the potential for Haki, King's Disposition as well as the other shades, but he never learned to actively, consciously use them as Luffy now have. I share the sentiment that "if he could use it, he would have been shown to use it" - Oda did, after all, show quite a fair bit of Ace fighting and it would have been easy to hint at him using Haki.
Yeah, Ace could've been shown using haki in the war. But if somebody had to think, hmmm, should I try to punch somebody with my fist using haki or should I try to see if burning the person with FIRE would be efficient in stopping him?