Rust is trying to reduce the amount of magic in the libraries as much as possible. This means that whatever the language/standard library authors have available to them, third party library authors should also have access to. `T?` would probably run against that, given the previous removal of `@T`, `~T`, `~T`, and `~str` in favor of `Gc<T>`, `Box<T>`, `Vec<T>`, and `String`.