As long as you don't use "*" as the version for any of the crates in your project's Cargo.toml it should stay at one particular version and therefore follow SemVer guarantees. If you are REALLY paranoid, you can even have the crate be a particular git commit or you could download your preferred version and then refer to it using the "path" key in the Cargo.toml.
Adopting crates into the stdlib would make stdlib and the crate depend on each other, which is not a nice scenario. Also having certain "core"-functionality be dependent on a 6wk release cycle is not great for stuff that is not good once it's set in stone (aka released on stable).
Real core functionality, bare bones stuff like parsing of Rust itself, I/O and TCP/UDP connectivity, which provide an unlikely to ever change framework for crates to build upon rightly has its place in std; e.g. convenience methods for downloading (like what reqwest provides) OTOH have to stay flexible to adapt to the real world.
