1. Yes, some mobs have retaliate ability just like furfur: every time every unit hits them, unit receives fixed amount of damage. Which means if you use triple shot unit like Bashira, on each her attack she will receive 3x of retaliation damage, and Nanaly would just insta-suicide. Rita and Ricola skill lets them not receive any damage in this case.
2. All units are attacked in order of placement, so they will not attack archers if you can put lighning rod nearby, but its not always possible. Some people had problem with machine-gun fish for example: Rita would be able to kill fish without getting fired upon. There are plenty of very nasty ranged enemies, like black goblins, demons, blue oni, golems and so on, and its not always possible to provide cover for ranged units.
3. Rita can be placed in far from main formation places, without support of healers and tanks. When enemy spawns ranged units, you pop her skill and she shoots at them without receiving return fire.
As every other gimmick unit, you shouldn't compare her to "general use" units, instead you should consider unique tactical advantages she can provide when you exploit her gimmick.
