The only reason is if a.) Zygarde 100% is banned, or b.) you are worried about Yveltal and Xerneas, whose aura abilities boost Dark and Fairy moves by approximately 33% respectively. Since you would likely not bring Zygarde against Xerneas, it only applies to Yveltal. Reason a is the only real reason, as otherwise the increased stats and ability to regenerate health are too useful to pass on. You only lose 10 speed, and gain enough hp for it to be worth it. TLDR: Only use Aura Break if Complete Forme is banned, otherwise use Power Construct, unless you really want to get rid of auras or want to keep 95 speed.
