Anime companies have a enormous amount of power compared to the manga creators (it's similar in the US when a novelist gets their book made into a movie) manga creators are afraid to make objections when a series is being animated for fear they will cancel it, do even more off canon stuff, and not get any more of their stories animated.)
Knowing that history has made a few people that have been reading manga nearly as long as I, actually refuse to watch anime adaptions.
I'm irritated with this episode because it was Studio Pierrot giving Kishimoto the middle finger. It's as bad to the opposite extreme as when Rei/Sailor Mars was fighting childishly with Usagi/Sailor Moon over Mamoru/Tuxedo Mask. In the manga Rei was cool and refined, they also trashed Makoto/Sailor Jupiter by making her a boy crazy girl, when in the manga she was a tomboyish girl who was the best fighter of the inner senshi, because she had a history of fighting bullies- including boys. Usagi wasn't a whiny brat in the manga either- yes, she was airheaded, but she had a bubbly personality that made people like her despite how dense she was.
Exactly and that pretty well my major gripe. Look at what happened to Fullmetal Alchemist. they pretty well had to redo then entire series because the anime team took liberties with the manga story.
Now Ill grant this much. Ive been reading Kekkaishis manga and just finished the first 121 chapters which conclude the animes 52 episodes. The anime team on kekkaishi actually imporved the manga with their verion IMHO by expanding the characters development and changing a couple sequences around to better flow in the story.
Part of my worry with this series was begun when they removed the part of the Hachibi fight where Saske thanks Karin for saving him. Its a key moment when Sasuke acknowledges her as both a comerade and thanks her which is something he never did in the manga at all save when he thanked Sakura which was an ambiguous moment at best.
So far peirrot has been good at keeping the filler in line with the mangas canon material, and at least making sure the filler flows with the manga unlike bleach where filler arcs come out of nowhere and have little to no connection to the main story. At least the Naruto filler can be easilly seen to fit. for example the investigate the sound arc to me is one of the few fillers that I personally consider canon capable.
I can see the need to use filler material but it should be used judiciously to enhance the story and maintain the flow of it like in kekkaishi. Not splashed around carelessly like a kid with a paint filled water pistol.