Being useful and hard to use are two different things. For developers, more buttons mean more things to consider when developing apps.
Personally, I just don't see the usefulness of Back buttons on WP. The Back button should be replaced when the multitask swtich feature in "Mango". The Search, maybe, it's useful sometimes. Again, like you've mentioned, the users would have to guess whether it will bring up the Bing Search or the in-app one. It sounds just confusing.
I'm not saying your POV isn't valid, but when you say "I just don't see the usefulness of Back buttons", my visceral reaction is as if you'd just said "I just don't see the usefulness of having thumbs." After using WP and Android for a while, hitting the back button became almost a subconscious reflex action, and trying to use something without it seriously almost feels like I'm missing a body part.
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.
Well, it actually did. Compare the Firefox 1 UI with current FF5 browser you will notice how much the UI has changes, simpler and cleaner, and the history-back buttons are gone.
I do not think those pictures show what you think they do.
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.
Obviously you didn't read my reply clearly. I said "the history-back button" (one of back buttons)is gone, that is the little down-arrow button when clicked it shows the history.
Personally, I just don't see the usefulness of Back buttons on WP. The Back button should be replaced when the multitask swtich feature in "Mango". The Search, maybe, it's useful sometimes. Again, like you've mentioned, the users would have to guess whether it will bring up the Bing Search or the in-app one. It sounds just confusing.