Indeed. Using plain structs one can't just add a single field with a reasonable default and still be backward compatible - which is a very very common case in A

Indeed. Using plain structs one can't just add a single field with a reasonable default and still be backward compatible - which is a very very common case in API design. You could add completely new revision of the API. But having to maintain it - typically additionally to the old revision since both sides might not updateable in atomic fashion - is a lot of extra work. Serialization which allows to add and remove fields in a backward compatible fashion is a mandatory requirement for any bigger sized project for me. If you need super high performance maybe use Flatbuffers or Cap'N'Proto instead of Protobuf/JSON/CBOR - but I really wouldn't recommend to go for plain structs.