WDDM driver is required for DWM. It was this way since Vista. The only difference is, Vista and 7 had the ability to render the desktop in the traditional GDI window painting method (non-composited) which relied on the CPU, however it was impossible to render many effects this way without issues. Windows obscured by other windows had to be completely redrawn when exposed.
With DWM, which powered Aero in Vista/7, the screen is rendered in a 3D space, composing each window as a separate texture, so to speak. Windows were handled separately in layers, keeping their contents in memory even when covered up by other windows. This requires GPU resources to perform efficiently, however, but it can be faster than the GDI method when decent graphics acceleration is present. Because of its nature, it allowed effects such as true transparency inside the window's space as well as thumbnails being generated for each window for the taskbar and overlaying windows didn't cause unnecessary redraw cycles. If no WDDM driver was present, Vista/7 had the GDI mode to fall back on (which displays the 'basic' or 'classic' themes).
In Windows 8, the GDI method of drawing the interface was completely removed. This is because the DWM composition is required for the entire Metro side to operate. Metro was designed around GPU acceleration technology. You got a lot of layering going on, screen-sized animations going on, and even apps that utilize 3D resources. Even though the Aero theme and its transparent features were removed, the compositing technology is still there even on the desktop. This means you can't get away with installing a Windows XP driver and having a desktop (albeit an old basic one like you did in Vista/7). If no WDDM driver is present, MS just uses the default VESA/VGA driver which is now coded to composite the UI using your CPU (software rendering). VESA/VGA driver is generally limited to lower 4:3 resolutions, such as 1024x768.
Because of this, you cannot rely on GPUs that are too old to have WDDM support. Even if a WDDM driver were made, the lack of texture memory or horsepower in these older integrated solutions would cause Windows 8's entire UI to perform terribly, if at all.