An SPA, if properly designed, can help interactive use significantly though, compared to full page reloads. You may pay for the reduction in interactive latency with an upfront loading cost, but when comparing to desktop applications that is often not too bad anyway (Excel doesn't load immediately on most PCs, games of any modern design generally don't either). People are more willing to wait a second or few initially than they are to wait half a second when getting more data from an app that they consider to already be loaded.
The problem is that many are not properly designed. In fact often the patten is often used when it is not really appropriate and a fairly static site would (again with that caveat: if properly designed) be much more efficient and just as user-friendly.
I agree, but it really depends on the context and if it is really properly designed (which takes a lot of effort).
Personally I don't mind waiting a couple of seconds for Gmail to load, but I hate to load that much for Twitter which is also an SPA. Not sure what the psychology is there.
The problem is that many are not properly designed. In fact often the patten is often used when it is not really appropriate and a fairly static site would (again with that caveat: if properly designed) be much more efficient and just as user-friendly.