@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.
![](https://i.ibb.co/DRD9TYH/King-sword-1.jpg)
![](https://i.ibb.co/2KzSB04/King-Armament.jpg)
![](https://i.ibb.co/RpgRRj3/King-sword-2.jpg)
The moment he shifted to CoA hardening in chapter 1032, he overpowered Zoro quite easily.
![](https://scans-hot.leanbox.us/manga/One-Piece/1032-014.png)
![](https://scans-hot.leanbox.us/manga/One-Piece/1032-015.png)
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.