Scenario:

1. I branch and start coding.
2. I arc diff.
3. For whatever reason, I decide I want to start with a fresh differential, so I abandon the existing one and arc diff again.
4. Down the road, I go to arc land and arc tells me that it doesn't know which revision I mean, and shows me both the abandoned and the current revision.

It's unlikely that someone will want to land an abandoned revision (and frankly, arc should warn in this case). In case of ambiguity, arc should not consider abandoned revisions.