What's so bad about having a declarative way of doing something that a lot of people will find useful?

And, how does one get sufficiently customisable incremental updates without having to standardise (and implement) a huge complex new language to control them?

By using element IDs. Every element in the incoming frame that has and ID that matches and ID of an existing element replaces it in the document. People have been doing this for years. It's called AHAH (Asychronous HTML and HTTP). Doing this without any custom code and with every update being as cheap as sending WebSockets frame would be absolutely awesome. It could be used to create very sophisticated and dynamic applications with very little complexity either on the client or on the server side.