Yesterday, I watched an episode of Good Eats, where Alton Brown made broth by pressure-cooking mirpoix and beef for about 50 minutes.
At the end of that time, he discarded the vegetables, stated that they had "given their all," and citing (as proof) the significantly higher level of liquid post-cooking.
This seems to me like a waste. Do these vegetables have any value left (texture, nutrition, something)?
On the other hand, he also stated that the difference between soup and broth can be the addition of chunks of the ingredients.