I understand that people are conditioned to think APIs are easier to consume when they're in the standard library, because that's true in many languages, especially those that lack a modern package management system (e.g. Ada). But it's not true in Rust and there are advantages to having libraries like `libc` outside the standard library. For example `libc` is updated more frequently than the Rust standard library (multiple times per month vs every six weeks).

Yes, there is. Not only is dependency transitive: you now depend on everything libc depends on — but now you're depending on the correctness AND properties of the dependency, to include security.