User blog comment:HQDominator/Highest Possible REC in Brave Frontier Global/@comment-26140304-20150924202402/@comment-26140304-20150925214248

Rec wouldn't really be needed on its own, but it'd be useful for conversion buffs. In a few days, Nadore will come to Global, who boosts Atk, Def, and Rec by 120%, which affects conversion buffs as well. Since Libera and Aurelia are often used together to combo with their buffs, the squad can reach to very high parameters when combining their buffs. Aurelia's 80% Rec to Def buff becomes more potent with Libera's Rec buff. If there was a Rec conversion buff, Rec will reach to even higher parameters, which would latter boost the potency of the Def conversion buff that Aurelia provides.

Why would you need this high of Def? Well, it's mainly because of the content that is getting increasingly harder. This is most prominent in later missions of RC5 where bosses like Esteroit will one-shot your squad even with a 140% Def buff and mitigation.