A good bag is highly recommended if you take your laptop on trips, or take it home every night. Laptops computers are frequently demolished in their carrying bag. The two main causes of demolition are poking the LC display and banging the edges. A good case has very stiff sides to spread out pokes, and lots of energy-absorbent padding around the edges to help when you whack it on the door jamb. Few cases actually have either of these features.
More laptops are lost to theft than damage, so camouflage is a wise too.