I have her AW. She's deployed 90% of the time. Lack of range is primary issue (AW slightly offsets this), since her attack is low because she heals UP TO THREE PEOPLE AT SAME TIME! Her effectiveness boils down to: "How many people can she cover at once?"
some numbers:
Marr, AW90: 343 attack (446 during skill), 235 range (286 during skill), Cost 17-14
Iris, AW 80: 498 attack (847 during skill), 260 range, Cost 20-17
Camilla, AW90: 572 attack (1030 during skill), 260 range, Cost 21-18
Marr's attack is basically multiplied by how many people she's healing (not accounting for over-healing). So by healing 2 people, she outheals both Iris and Camilla outside of skill. With all 3 skills active, she out-heals Iris and comes up 138 short of Camilla. Healing 3 people, she wins easily (she's 1 point shy of Camilla during skill without Marr's skill active). She's also cheaper than either of them, and out-ranges them during skill. Again, this does not count for over-healing, nor for the need for single-target burst healing (which you wouldn't use Marr for anyways, she's a multi-target maintainer)
TLDR: Marr is probably the best general-use healer on Nutaku Aigis atm. The only situations you would use a healer over her is burst healing or when due to deployment slot spacing and/or enemy wave patterns, she won't be able to make effective use of her multi-target healing (she'll only need to heal 1 person 80% of the time, etc.)