In PHP-1222, a user expected that the "majority" write concern would only consider data-bearing nodes. scotthernandez provided me with a helpful explanation for why it needs to count all nodes, as the goal is to avoid data loss due to roll-back. It would be helpful if we explained this with a blurb in the documentation.

http://docs.mongodb.org/manual/core/write-concern/ talks about "majority" persisting data to storage nodes, but it doesn't address arbiters at all. Obviously, they don't hold data, but they are counted in determining the "majority".

He also clarified that "majority" means something else when referring to election voting (it depends more on voters and vote weights than physical nodes), but I don't think we need to clarify that with this ticket.