You can't really use $out with command cursors. Although you do get a cursor description, iterating over it returns nothing (as the result is in a different collection). Even if we force it, you can't get to the results with a cursor, so people really should just use "command" in this case. I wouldn't want to change read prefs on users for something that doesn't work anyway, and the servers gives a nice error that results in a "not master" exception anyway.