Arm's Length will still cause diminishing returns. Let's say you get hit by 1000 10 times for 10000 total damage.. Ramparts on its own reduces that by 2000, Arm's Length on its own reduces it to 8 attacks meaning it reduces incoming damage by 2000 as well, resulting in 8000 total damage and then adding Ramparts would technically lose value and reduce 1600 damage. Not disagreeing, just clarifying something.
Arm's Length is a form of mitigation since slow effects reduce incoming DPS. Many non-tanks and sprouts miss this part, but when trash pulls hit you with Arm's Length it applies a slow to reduce their attack rates which is a very good form of damage reduction.