In addition to being able to come up with a good dynamic pose, and that can be directed by the artist in a pinch, the ability to actually hold a pose is vital for a good model. It is harder than you might think to hold even a simple pose. Try holding a pose for twenty minutes without moving at all, then take a five minute break and resume the pose exactly as you were before. Repeat for several hours...
It's pretty gruelling. One of the models I work with is particularly good at always just falling back into the same pose without any coaching. I complimented her on it once. She said with a rueful grin, "It's easy! I just move until I find the place where my muscles ache the most, and that is the pose I have been holding."