And what advantages has that powerup aside from visual aesthetics?
Coating yourself is not really needed aside from some specific moments (for example, Zoro not being able to block in time an attack with his swords, the attack will reach his body so he needs to coat himself).
On the other hand it has some disadvantes -> the permament coating.
-Spending Haki for no reason continuously -> that's why people don't go full Haki coated during all the combat, aside from G4 Luffy but that is only for him.
-Lower "mobility". Like it was implied with Luffy, G4 armonizes his rubber with Haki, he IS STILL rubber but a with a better "quality", meaning in his other forms he lost the rubber properties (stretching) while coating himself (well, there are some inconsistencies here, but more or less that's the point). And this is another reason for not using full body coated all the time and only activating it when needed. Coating weapons on the other hand does not have this disadvantage.
Idk, that seems more like a fantasy of a teenager.
Zoro had enough powerups this arc, it's kinda ridiculuous to have even more. He should be able to take King with what he already has.