I think this only solves the problem for the API owner, not the consumer, right? You can't use cURL with protobufs like you can with REST to quickly play/interact with the data
There is tooling built to easily do grpc requests from the command line[0] or even a GUI if that's your thing[1].
The tooling isn't quite as ubiquitous as cURL, but if anything I would think that its even easier because you have introspection into the contract with a .proto file.
[0] https://github.com/fullstorydev/grpcurl [1] https://github.com/uw-labs/bloomrpc