With one end in Python 2 and the other end in Javascript, using binary protobufs seems misplaced optimization. It's nice to know the support is there (well, not in Python 3, apparently), in case you need to talk to something that speaks protobufs.

I'm looking forward to seeing protobufs in Rust as a macro. It should be possible; there's an entire regular expression compiler for Rust as a compile-time macro, which is a useful optimization.