> The best use case I can currently think of is to get rid of the geometry_msgs/FooWithCovariance message types and merge them into one geometry_msgs/Foo message with an optional covariance field
While I agree that there may be valid use cases for optional fields, I don't think this is one of them. In a different robot framework that I regularly use, there is no `PoseStamped` message, only a `RigidBodyState`, which is the logical conclusion of what you propose and which has these fields:
- position, cov_position
- orientation, cov_orientation
- velocity, cov_velocity
- angular_velocity, cov_angular_velocity
In contrast, I like the ROS messages (`Pose`, `PoseStamped`, `PoseWithCovariance` ...) better. The signature of the topic tells me immediately which fields are used or supposed to be filled. I don't have to do a `rostopic echo` or read the code to find out, and if they later decide to use a covariance, the API changes, so my code breaks and I know I have to adjust it.
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