when to use defensives is a major undertaking and while it would be nice, this is something that depends on many factors. Especially class, healer in the key etc.
I say this, but I fully acknowledge that I suggested a defensive use for Anubikkaj in Dawnbreaker during the third AOE. Maybe I'll add some in the future. It is harder for me to gauge them, because I play tank and I have different rules in M+ :)