I think what's funny about the monster arc is that despite its name, its most important function in the story had very little to do with monsters, it was an arc to turn the main cast into a united group of friends, it made the twins relevant. After this arc you could believe that anyone of these people would fight to protect the other.
Prior, the connections many of them had where superficial, we knew Michael was training some of them but that was just a few lines, to us the readers it meant very little.
At the same time I get some of the issues people would have had with it, it put a pause on some of the developments we where getting invested in, the MC didn't have a single good fight. But it was far from bad, I still really enjoyed it and I can see its value to the story.