Maybe.

Protobuffers or Thrift can definitely be smaller in byte size, which does matter somewhat, but JSON right now is just more easily accessible.

I think in a few years, assuming JSON logging really takes off, you'll see people using these other RPC formats in order to optimize and reduce the size of their messages.

I view 'better' RPC formats as a performance optimization of the same basic idea though: Easily add new fields, and use a format designed for computers to consume.