I have read the documentation provided by MongoDB here, but I fails to explain why we should set non-primary DC secondaries to priority 0. http://docs.mongodb.org/manual/core/replica-set-architecture-geographically-distributed/
I understand that if I have a setup with two nodes in site A and 1 in site B, the node in site B can never be primary if A goes down, since it no longer has a majority. So that cannot be the reason why we set priority 0. The only reason I can think of is to avoid any "ties" in voting if the primary in site A goes down.
I have read this question here which is similar to mine, although the OP seems to want automatic failover to site B, which I fully understand that I can never have. (Why do MongoDB docs recommend/insist on specific replica set priority configurations), but the answer just list some scenarios, I doesn't explain why (or I'm still not understanding).