I believe that DWM is necessary for most of Vista's special effects: blurred transparency (idk if windowblinds uses its own method to do that now), window animations/transitions, win key + tab 3d window switching, dreamscene (maybe?), window previewing (when hovering over taskbar items), etc. If you don't need or use these things, then I don't think there's any problem with disabling DWM although it may break some programs that use it. In fact, you can't run DWM without a DX9 gfx card anyway, so it's automatically disabled for those systems.