So what is the preferences now a days when you can go with pushState and not mess up the SEO stuff.

View caching or just keep everything on the clientside?

With tools like Backbone, SC or Cappucino... are there any reasons not to use them other than programming an API from scratch and "duplicating" MVC on the front-end, meaning more work. I mean I can see that if you got more than one "ajaxy" thing going on, Backbone could be pretty useful.