Robert's raw healing power is just too low. Robert AW80: 395, Fedora AW80: 519 (Robert CC60: 311 / Fedora CC60: 445). And frankly, as far as third healer is concerned, even Alissa (cc50: 378!) would do just nearly as well, and for the fraction of resources spent
Besides, Fedora's Increase Range would sometimes allow you to cheat your way out of needing second healer to cover another side of battle formation or bring another damager to the chokepoint, and AW ability gives her faster deployment. While Robert's what, more booze?)
Even in skill awakening front Fedora gains skill to heal two units at the same time, while Robert loses healing to become an amazing high-powered physical man-witch XD