Replica set config documents look like:
The "_id" field in member subdocuments isn't useful in any way I know, but requires extra typing and it must be chosen to obey certain rules (it must be unique and between 0 and 255 inclusive). If we handle it incorrectly it can cause bugs (
SERVER-40355). In summary, the _id field has cost but perhaps no benefit: uniqueness is provided by members' addresses and order is provided by their array position.
Let's consider deprecating _id, and eventually ignoring it completely.
Since this has diagnostic value, we'd prefer to change the type of the "_id" field to avoid introducing bugs.