Well, any good photographer experienced with shooting models will tell you....
Models also need to be able to act...
Yes, there are models who specialize in runway that have some particular skill in that area over an actress fresh of their latest movie set, but...
I can't tell you how frustrating it is when you are on set and have a model trying to look sexy when she's supposed to have a lock of shock, surprise, fear, loneliness, etc. Every model has her "magnum" pose and it gets old fast. Pros need models who can get a grip on what the photo is trying to sell - whether it be a product, service, idea, or emotion. So can an actress replace a model? Most definitely, provide she looks suitable for the project. There are certainly a lot of beautiful actresses and actors, and you can bet that they have to act sexy once in a while and can do it just as well in front of a still camera as they can in front of a movie camera. So for all you potential models out there. Don't laugh when the photographer asks you to be serious and look a certain part, or you might not get called back again. Acting school is always a good thing too, and who knows? Maybe you'll get an acting gig down the road too.