Merc Defiance Aura - Underrated??

I had never considered using an Act 2 merc for Defiance aura, just under the assumption that Defense is so much less important than other attributes, so the aura is probably a bad choice.

Starting running it recently on my Firelock, and I like it. It's really the only Aura that helps him directly, versus helping my merc and demon. I had been running Prayer, which technically helps him also, but with Hex Siphon and Enigma, I'm getting like 40+ life per kill so Prayer is pretty meaningless in comparison.

Brought my defense to around 6500, and chance to hit for a TZ Dark Lancer went down from 75% (if I recall) to 45%. That's a pretty meaningful reduction against physical damage, 40% (1-45/75)! Yes if I'm running it doesn't work, but that's still pretty dang good. I feel like I do notice a difference against some T-5 P8 Herald physical damage packs that hit like a truck.

Swapped out Treachery for Fortitude on my merc to better take advantage, and he is now running 15k defense. Chance to hit on him should only be around 25% @15k.

Also, I randomly had a Horker Barb tailing me and scavenging me today. Was a pretty decent combo, lol. He used Shout, which took my merc defense up to 35k! (~12.5% chance to hit)

What Act 2 merc aura do you guys think is best for non-physical damage characters?