It's nice to have a form for that value that you can to say to someone without having to explain. Saying "the empty list" sounds awkward, and it would be consistent with pg's design goals for Arc to make it simple to talk about that value.
That's a fair reason, though you can just teach people to read '() as 'nil' (say in the tutorial).
I'm not second-guessing the decision; I just want to confirm that I'm not missing a more substantial reason. From an implementor's perspective, it's good to know that nil-termination isn't a reason to require a 'compiler pass' for arc. Even if you include 'nil to be eq to '(), ac-denil and ac-niltree are redundant. It seems a strange decision to perform the translation when much of arc is so barebones and no-nonsense.