I think the Windows Phone team deserve some credit for doing this, knowing that the developer reaction to it will be deeply negative (read: most comments in this thread).
The simple fact is that it works better for Windows Phone users. If developers hadn't been using webkit-prefixed CSS everywhere it wouldn't be an issue, but it is.
That said, I wish Windows Phone IE had better developer tools. Both Android and iOS let you plug your phone in via USB and access the web console. It's great.