isMaster now includes "active" nodes, in "hosts", arbiters in "arbiters", and passive nodes (priority 0 but non-hidden). It'd be helpful to also include hidden nodes. The use case we have in mind is for automated rolling index builds; our replset configuration consists of 5 active data nodes, and 1 backup node running on EBS (that we take EBS snapshots for). The backup node is hidden, so we have to call replSetGetConfig to discover this node. It'd be nice if isMaster just included that, since we could presumably just query the driver (which obviously calls isMaster regularly).