[ebidel]

It's a common misconception that Shadow DOM is required to create a "web component". What it brings to the table is style and dom encapsulation. Not using it means components won't have those benefit (by default), but there's nothing stopping people from creating custom elements that are not encapsulated.

An analogy can be made to requestAnimation. raF isn't required to create animations, but it's highly useful in that context. raF can also be used for other use cases outside of animations (scroll effects, etc). Shadow DOM is the same IMO. It's a useful standalone technology, but when used in the context of web components, it really shines.