Essentially my argument boils down to this:
- The favicon is useful in general
- It would be nice to have it appear in a consistent, fairly prominent spot on the screen
The current tab is often in a part of the screen with relatively low visual focus, and is not in a consistent location. This can be pretty extreme — for example, right now in fact, the favicon for this page is tucked away in the very bottom left corner of my screen, somewhere my eyes don't tend to travel, and certainly not for navigation/orientation.
The favicon is part of the page identity, and is often used in conjunction with the URL. In the same way that looking at the path indicated by the URL can help ground your location within the structure of the website, the favicon provides a visual (non-textual) indicator of what website you're visiting, and those purposes go hand-in-hand. Therefore it makes sense to put a favicon near the URL.
Additionally, nearly every other feature of the web page and browser and application and operating system that is relevant to the question of “what am I currently looking at” is at the top: the website header, the URL, the name of the application, the menus, the navigation buttons, and more.
(but also it's not that big of a deal)