As part of this task we should document on the project page why we choose core and not an extension or gadget (or why we first chose core and then moved to an extension, if that happens).

Personally, I think that it's okay to add things to core that are a) able to be used by extensions, and b) useful to every MediaWiki installation out there (even if it has zero extensions installed). Of course, it's fine to have dependencies between extensions (and we do that often) but it's nicer to not have to.