Imagine two nodes, both initiating updates to the same row. One has observed a number of node additions and deletions. The other has not. They will perform their updates on disjoint sets.
> There's not really any way that nodes can disagree on this.
Atomic, dynamic updates to a ring are a difficult thing to do. If I'm missing some additional mechanism that ensures agreement on ring membership, please explain.
Given that nodes cannot disagree on the set of replicas for a row, quorums must (by definition, and the pigeon hole principal) overlap by at least one replica.