He needs to defeated so Oda can breathe some life back into his character. Part of the charm that made Zoro so appealing during the preskip was his desire to become stronger. He has a goal that he wants to achieve, and we got to see him make an effort and acknowledge his shortcomings to motivate himself to want to do better and achieve that goal.
This is one of the reasons I invested so much into his character preskip. You wanted to root for him and see him succeed, because the path laid before him was a difficult one. Mihawk made it a point to let us the readers know that it wasn't going to be easy. Then Zoro goes and trains with Mihawk for two years, and all of a sudden, that path is no longer being shown to be difficult.
Mihawk said the path to become Pirate King is even more perilous than surpassing him, but for christ sake, the way it's being treated, the path to become Pirate King is thousands of times harder. Where is this difficulty of surpassing him that Mihawk spoke of? "The World is vast, go out there and experience it." What did Zoro get after than that actually pushed him to his limits? Mr. 1, Ohm, and Kaku? Surely these can't be the Swordsmen that Mihawk wanted Zoro to experience.
Mihawk fought every swordsman in his path to become the strongest. Zoro hasn't fought a single swordsman worth a damn since post skip started. Not once has a powerful New World Swordsman been put in his path for him to experience this vast World. If Oda doesn't rectify this soon, then all hope for the character will be lost, as it will turn out that the only thing you need to do to become the World's Strongest Swordsman, is to be trained by the World' Strongest Swordsman, and then defeat him.
So in my opinion, Zoro needs to suffer at least one more defeat to get him back into that mindset that he needs to push himself to become stronger, so he himself surpasses Mihawk upon his own merits, and doesn't just surpass him because Mihawk trained him to do so.