If you want something private, don't put it on other people's platforms, it's very simple.
Yes it matters, there are use cases if not only for privacy focus people. Why would the hold the keys? I actually have found a good example of one that I am working to verify.
The chat is a bit more transient but you're transferring messages encrypted again over websocket for example, then decrypted on client.
I'm not seeing what the blocker is, or even issues with scale given its the client side doing the work.
Working example : http://securinote.com/#/notebook/bf528a8d-bf34-4f49-8cb9-e63...
key : embedding
Open up a second tab, make a change, check the network you'll see its sent over the wire encrpted via websocket.