Indeed - I like the concept of debuffers, but unfortunately when you can only deploy so many troops at once, you need to prioritize the essentials. And too often, I barely have slots for the essentials.
As for Kanon, however... I've managed to get 11 copies of her! Is that enough? Probably not, if I wanted to max her skill and mincost her... but it's pretty good. Now I just need 24 Platinum armors to fuse the 8 spares I still have...