The only reason I could think of to use a conditioner after a deep conditioner is for the 'cones, conditioners contain, but deep conditioner may not. The 'cones may make the hair smoother and more easily detangled than using a deep conditioner alone. For instance, in L'anza's Dry Hair line, they have a moisturizing shampoo (step 1), moisture treatment (step 2), and detangler (step 3).
The moisture treatment is the deep conditioner which leaves the hair very soft and conditioned and the detangler contains 'cones which makes the hair smooth and detangled. Perhaps putting 'cones in deep conditioners would prevent penetration of important ingredients.
That's just my take on it.