>> There's a place for it within SPAs - you want the browser back button to retrace your path through screens in the application, not exit it, unless you are already on the first page.
No, You SPA should have it's own back button within the app. My browser back button should get me out of there no matter what.
How could the user expect consistent behavior from the back button if web apps can no longer leverage it like web sites?
I use a couple of web apps that have this. It's frustrating, because muscle memory has me clicking the browser back button instead of the back button in the app. So that probably takes me back to the front page of the app, or out of it entirely, which is not what I wanted at all.
Then you lose the benefit of having the current URL actually linking to the resource you are currently viewing. If the app is not properly making use of URL modification and the history API, getting back to where you were requires navigating the whole maze each time.