"legacy" functions vs rolling our own

If these were going away in the future, they'd be deprecated, which is not the case. They are "legacy" simply because they are inherited from a past time that didn't utilize PHP's own session APIs.

Our manual encourages you to use the $_SESSION superglobal simply because that means less overhead and that if you know how to use a regular PHP session, you also know how to use one in CI - there's no point in learning to use new APIs if you don't need to.