Would be interesting to see some simple benchmarks comparing a base JSON only Rails app with Rails::API.
"We are building an API. Rails has extra components which aren't used but might cause overhead. Here is how to remove it."
I think that's a valid enough use-case.
The README itself says Rails::API is faster, so not sure why a simple benchmark to back that up isn't a good idea.
edit: It really boils down to another way to visualize the overhead vs just listing what's been removed.