I swear these guy don't read the manga.
mihawk asked Zoro, if you break your swords before;
Zoro said yes,
Then mihawk said, you should learn to coat your sword with haki, so it won't break. That's the all reason why he should learn Haki, to protect his sword, if Zoro already knew how to coat his sword with haki, he wasn't going to teach him anything.
Roger can pummel people with Haki blasts, Shanks can damage a ship with CoC Haki and split the heavens.
mihawk can't do shit when cutting doesn't work.
Evidence 1
Evidence 2
He uses Buggy's own attack to beat him
VS.
Roger's first attack with sword was pummeling with advance CoC + CoA.
Shanks, just like Roger is on a different level than every swordsman.