There's a big difference between drawing an idealized nude woman, and drawing a 30-year-old, 200 lbs., nude woman. If you produce a representative image of the latter, you can really draw.
I've done some good photos of cats, but babies don't pose as nicely!
"because then the students would have to attempt to draw what they saw, rather than what they imagined. "
Precisely.
And yes, babies tend to move like washing machine agitators with arms and legs when you try to take a pic.
Or they sit still, mostly, but they look everywhere in a constant skull movement not conducive to good useful photography.
*chuckle*
Not only that, but you've got a stomach of iron, too!
Well, even if the model is an attractive lady, drawing what is there, rather than what is in your brain can be tricky. My opinion is that you should generally use models that are closer to the ideal, not grossly overweight or unattractive ones. Getting a likeness isn't easier with the body than the face, I don't think.
Sometimes it can be easier to draw the heavy ones, since the actual structure is hidden. Plus, somehow there is more at stake in getting the attractive ones right, so one tends to be less relaxed, which is a hindrance.