If you'd like to elaborate in a way that would be highly visible to 18F, feel free to weigh in on the issues page[1].

I will point out that JSON is recommended as a default, and if there is a compelling reason not to use it in a specific case, that should be fine[2].

[1] https://github.com/18f/api-standards/issues or https://github.com/18F/api-standards/issues/8

[2] https://github.com/18F/api-standards#just-use-json

Not necessarily: http://json-ld.org/