[cbsmith]

In practice, to have decent compatibility as revisions changed, you really had to minimize the use of "required" fields anyway. While I agree it was sometimes nice to be able to avoid having to worry about it, in practice protobuf parsing imposes a very minimal set of constraints on data types. A successful protobuf parse was not nearly enough to ensure you had data integrity. I've run in to more than a few cases of developers using the wrong protobuf (v2) definition and not realizing their successful parse was still wrong.