Communication. The new guy joining your team can be immediately productive with JSON, protobuf, etc, but will take some time to understand the homegrown format.

Communication. The new guy joining your team can be immediately productive with JSON, protobuf, etc, but will take some time to understand the homegrown format. Also much more error prone. Most teams don't want to be writing binary encoders and decoders as that's not where their expertise lies, and would much rather focus their time on business features.