CoA / Tekkai is important if you have the base strength, and CoO is important if have the combat skill or raw speed.
CoA user Marigold lost to pre-ts Luffy's G2 bazooka and CoO user Sandersonia get blitzed by Pre-ts Luffy's G2 speed. Rayleigh noticed pre-ts Luffy has base power to defeat even skilled Haki users so if they add Haki to that base, Luffy's powers would increase dramatically which is why they focused on Haki training.
A lot of marine who are not even vice-admirals uses CoA as we see them going to stop birdcage, even Tashigi uses it, Kuja fighters, some pirate named Raisin in Big mom pirates, all of them uses,
But only makes it a big difference if you have the base strength, its very important for someone like Sanji or Franky, but its not very important for Usopp and Nami.
Franky also have different weapons other than base strengths so he compansates that ability with science.
Monet defeated Haki user Tashigi.
They can still be formidable without learning CoA, and can beat some CoA users like I mentioned above.