The use case I'm most excited about for Web Components are those producing reusable components (like YouTube embeds you describe). Building the same components for different frameworks no longer makes sense when you consider that you could publish a web component instead that would work out of the box with most modern frontend frameworks. I'm incredibly bullish on them now, and we're going to be turning Ionic components into WC's so we can work with all frameworks people want to use (because, there hasn't been one single framework that has "won" the frontend market)