I'd go with `cerr << request.DebugString() << '...' << response.DebugString();`, preferably with your favourite logger instead of `stdout`. My browser does the equivalent for me just fine, but that required an extension.

I buy the familiarity argument, but I usually don't see the wire format at all. And maintenance-wise, protobufs seem easier to me. But that's because, e.g., someone set up a presubmit for me that yells at me if my change isn't backwards compatible. That's kind of hard to do if you don't have a formal specification of what goes into your protocol.