Celebrities or Models?

While rewatching The September Issue yesterday (a fantastic documentary on the inner workings of Vogue), I reconsidered a topic I've often overlooked in the past. Celebrities are often found on the cover of magazines all over the world. I'll sometimes buy a magazine I've never read before just because a favorite celebrity of mine is on the cover. Apart from simply endorsing and being the spokesmen and women for products, celebrities have also become some of the most sought after models in today's industry. However, they were never trained to model. They don't have the modeling resume that those who have worked for years at various agencies hold. So why use them as models? Well, for sales obviously. Some models even go the opposite way and become movie stars (see Rosie Huntington-Whitely in Transformers 3).

What do you think of this? Should celebrities stick to what they do best and give models a chance?