As one example of why this matters consider the SockJS protocol that provides HTTP fallback options for WebSocket.
The protocol does not define any client-side "close" frame. It expects the server will detect when the HTTP connection is closed and indeed a variety of server-side implementations in a range of languages (node, erlang, python, ruby) do.
A Java implementation – with Servlet 3 async for the long polling or HTTP streaming – however cannot detect when a client has gone away. This is a very fundamental, missing feature that puts Java Servlet-based implementations at a significant disadvantage.