A critical issue.

Since we were talking about filler episodes, let's abstract from this discussion and the specific title – I've seen online countless times the opinion that filler episodes in series or cartoons are bad and don't deserve to exist.
Of course, I don't agree with that.

Similarly. I've always seen great value in filler episodes, as a way to develop characters. The main plot usually doesn't do a good job of this, or rather, I haven't seen it do a good job even once.
As for the second season of Wakfu... Well, if it weren't for the filler episodes, it would be extremely weak, because the main storylines with Kilby and Rush are just terrible (the storyline about saving Grove is okay).
Overall, I still agree with Ples's comments about this series (both positive and negative).