> When either an SPA or a multi-page site is done well most users can't tell which sort of site they're looking at.

And therein lies the problem. In the _vast_ majority of SPA sites I've been to, they are not "well done" by this definition. It is commonplace for things to break, like the back button as the quintessential example, because the developer(s) didn't spend the time to make sure things work correctly. With a non-SPA site, you generally have to go out of your way _to_ break those same things.

I like SPAs for some things (gmail being a good example), but they should not be the default implementation; there should be a _very_ strong argument before the SPA architecture is used.