I am well aware that we all got 10 fingers unless you are mutants. Why use only one, right? Well, when comes to UI, the less is better for users. And it's less UI behavior consideration for developers. That's all.
So you would remove the back button in web browsers, right?
FF1 http://en.wikipedia.org/wiki/File:Firefox_1_0_8.png
FF5 http://en.wikipedia.org/wiki/File:Firefox5.png
Remember Netscape? http://en.wikipedia.org/wiki/File:Netscape_Navigator.png
I can clearly see the back button in all 3 UI pictures, including the one for Firefox 5. For that matter, I can see the back button in the Firefox 5 window I'm using to type this comment. I agree that Firefox 5's UI is simpler and cleaner than Firefox 1's and Netscape's, but I don't know where you got the idea that they'd eliminated the back button.