What percentage of projects need them? Surely it's not a core requirement outside of actual real-time apps (and not 'lets play with realtime on my shiny blog').
I think any app requiring multiple user seeing the same screens could benefit from them. Compare an open websocket versus polling every 30 seconds for the use case of changes made by other users of your system. Instant notifications and changes of your ui in real time versus a delay that could lead to users trying to perform operations on outdated state.
WebSockets are one of the most underestimated value-adds in modern browsers. Pub-sub. Real time. Ease of data transfer and client-to-server synchronization. I can't think of a modern project that wouldn't benefit from sockets, unless the site is literally 100% static. But then, why use most of the stack outlined in the article?
