Exactly. The core can't contain everything everybody would ever find useful. Instead, it's the core's job (IMO) to make it easy to define those things as they come up.
Additionally, it may or may not be the core's job to encourage canonical APIs for non-fundamental but really useful stuff, just so that programmers don't end up defining the same things over and over in ways that are flawed or incompatible with each other. I think this "may or may not" issue is the R7RS split in a nutshell.
In my own opinion, a "core" that tries to provide a comprehensive bookshelf of useful but programmer-definable things isn't a language; it's a utility library. Some libraries may be extremely popular, and they may even be standardized, distributed, and/or documented alongside their languages, but it's ultimately up to the development team to choose the combination of languages and libraries that's right for the project.
