That's exactly what I would want along with showing Windows 8 Store Apps on the taskbar -- both as options that the user could enable/disable. For me that would also show they are more committed to the desktop by enabling one it's best features from within the start screen.
Meh, why? If your program is pinned to the taskbar, you don't need to use Start to open it. If it isn't, you don't need to see the Taskbar.
Plus it would have to support being docked at any edge, like the desktop does today.
Plus the default bottom edge is used in Metro mode to enable secondary actions.
Plus 8.1 has introduced a mode that allows you to pull the tiles down and display the list of all applications. Where does the taskbar go if it's docked at the bottom? Where does it go if it's docked at the top?
Plus the taskbar uses a different UI paradigm, where hovering and right-click menus provide actions.
Plus the taskbar shows an icon to enable the desktop on-screen keyboard icon, when using a touchscreen; if it takes you back to the desktop, wouldn't that be counter-intuitive? If it brings the keyboard up on the Metro side, that doesn't make sense unless you can type into something (and Metro apps do this automatically).
Plus consider that the taskbar can be resized to take up half the screen. Someone, somewhere, actually does this.
I not only think this isn't useful, I think it would be a mistake.