Having just spent the past two days reverse engineering an API that used protocol buffers, I can tell you I would have much rather done it against XML.
Why didn't you have the descriptor files? Optimizing for ease of external reverse engineering seems rather counter to the authoring organization's actual goals.
If you want third parties to be able to interoperate, give them the descriptors you use.
As for parsing, I guess I don't get what the big deal is.
Writing lots of code to read and parse data types as text, especially when no schema is available, is time consuming.
Protobuf is necessarily validated. The input types are known-correct, conversion to the domain model is direct and straight-forward, and it's exceptionally easy to generate code or otherwise automatically deserialize to native representations.