@ConquistadoR Half of the time when Sanji uses CoA, his leg is on fire and the other half, he's wearing black pants, lol. Like, see this:
Also, there just hasn't been a big enough distinction between 'normal CoA' and hardening. Like both of things have never been talked about separately.
Like take King's case - he was matching Zoro equally in swordfight when he was using the invisible form of CoA.
The moment he shifted to CoA hardening in chapter 1032, he overpowered Zoro quite easily.
IMO, one doesn't necessarily need it to be strong - characters can be really strong without it. But if you have it, then using it is definitely a better option than using basic invisible CoA.