Earlier quoted context omitted.
Mail is not a good example. Why would you like to read a collection of documents through A Single Page interface? Gmail was a fantastic improvement over Hotmail and Yahoo, and it provided UX innovations we still haven't caught up with, yes, but MPAs are naturally more suited for reading and composing them. Overriding perfectly clear HTML structure with javascript should be reserved for web experiences that are not do…
Mail is good for a SPA because the main central view which shows the different items (emails) to view or take an action on is based on a resource intensive back-end request, so keeping that state present and not having to refresh it on many of the different navigation actions yields a tremendous benefit. You could do some client side caching with local page data, but just keeping it present and requesting updates to…
That is, it may seem a fine optimization, but has led to a fairly bloated experience.