> Philosophically, checking that a field is required or not is data validation and doesn't have anything to do with serialization.

My issue is that people seem to like to use protobuf to describe the shape of APIs rather than just something to handle serialization. I think it's very bad at the describing API shapes.

I think it is somewhat of a natural failure of DRY taken to the extreme? People seem to want to get it so that they describe the API in a way that is then generated for clients and implementations.

It is amusing, in many ways. This is specifically part of what WSDL aspired to, but people were betrayed by the big companies not having a common ground for what shapes they would support in a description.