I think this is a good decision, from Microsoft's POV. If Windows 8 is to succeed on tablets as a competitor for the iPad, it must be possible to interact with it as much as possible without ever leaving Metro. As much as I dislike Metro on the desktop, on tablets it's the desktop UI that should be avoided. And having a tile for the regular Windows Explorer on the metro start screen by default just isn't a good way to achieve that.
Those who need Explorer can still find it easily enough. And add it to the start screen if they want.