A counterpoint: Babel's extensibility doesn't matter in practice at all, other than helping the Babel team organize their code.
Pretty much every new ES6 feature required parser and babel-core changes just to be able to be used.
Example: a lot of changes that only worked in Babel 7 (that was on Beta for months) were not possible in Babel 6, and so on for previous versions. A plugin was not enough: you also needed parser/core changes.
Other than for novel non-standard features (like code substitution), plugins are not exactly that powerful, and even things like that are frowned upon in most environments, as 99.9% of people just want ES6 features.
