Actually, sometimes I use both the Type Cover and the Microsoft Universal Mobile Keyboard at the same time, particularly when I'm in Photoshop or InDesign. The reason being is I find that the SHIFT key will stick if you hold it down too long on the Type Cover 2 (requiring tapping it to "unstick" it). So for SHIFT, CTRL, or ALT operations, I use the bluetooth keyboard.
Another time to use the keyboard is if I want to have the Surface machine being in portrait mode. And the last situation is when I need the keyboard to be wireless (detached).