"they stay still longer than a model does"
yes, I had always been concerned about models getting uncomfortable holding a pose, so I worked often from photos as well. However, nothing beats a life model for drawing exercises that must be done quickly to capture the dynamic of the pose with a few strokes. Such quick sketches are very useful for the final painting.
And then, you might enjoy the interaction with the model, as in this image by Kurt Regschek:
