This isn't really about "getting the latest version", it's about "getting the latest security patches for a stable version."

The presumption here is that you're running on a stable base image, Debian Stable or Ubuntu LTS or something. Updates to system packages are therefore basically focused on security fixes and severe bug fixes (data corruption in a database, say).

Even if you're pinning everything very specifically (and you probably should), you still want to at some point get these (severe-bugfix-only) updates on a regular basis for security and operational reasons.

Blind reliance on caching prevents this from happening.