No doubt MVC facilitates all of the above, if you favour strongly typed, then that's good, continue that way, but its not necessary ( controller is now basically going to be a restful web service )
"We can already serve JSON or XML data off controller endpoints in MVC3" - true, and you'll probably be using Web API to do that soon.
I am just saying its not necessary to mix, I feel it over complicates the client by making you mix serverside logic a la razor, webform whatever.
I don't particlarly see the point in it now though. If you look for example at the knockout mapping plugin for knockout js, it will take a json source and automatically resolve into your js viewmodel. so on the client all I am doing is thinking about the client. give me a rest api and some json objects and I am off and running.
All I am simply saying, Pure Html/CSS/JS can be done without the need for any knowledge of the server on client side, just the interface contract and like I say a restful wcf service / Web API is good.
I am not an "out lier" when I say I think its a good way to develop, its my opinion. The truth is, I did refactor a load of "webforms" lately and the result was,
1. Html/Js/css - knockout.js and amplify.js
2. JSON Service ( c# in behind serving the data and validating business rules )