Diversion is a fantastic supplemental skill and was paramount for a lot of my victories as a Viking/Warrior in my first play through. I don't see why put any exception for bears.
(Maybe deep down I hate them too... traumatized by too many fights in Bolo in Armored Princess).
As you said, diversion is based on luck. While I think it is a good skill, it is hardly a good use of early runes as I prefer to use runes on hard guarantees, not hoping it will work on a bear for 2 turns.