I feel like Battle of the Abyss was the first of many novels that were simply used to take up space. Theres just so many ridiculous filler novels that you have to wonder, with all the success and attention they had creating certain novels, they just decided to go the other way.
I disagree that any novel is a filler. This is the authors chance to truly flesh out the heresy. I have never understood the mindset of rushing a story we know the ending to. Nothing wrong in my opinion with fleshing out all the legions.