Do pressure-cooked vegetables really have nothing else to give? 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.
After extended pressure cooking (or any liquid cooking, for that matter) the vegetables' cell walls (composed primarily of flavorless starch) have broken down to the point where they're barely holding together, and are basically mush.  Most of the flavor and nutrients contained within those cells has escaped into the liquid at that point.  There might be a bit left in the solids to extract, but very little practical way to do so. 

It's probably more accurate to say that the vegetables have given up enough that they're unpalatable. You could eat them, but they wouldn't be anywhere as pleasant as fresh vegetables, and would offer little nutrition separate from the broth itself.

I suppose you could puree them (though you'd be turning flavorless, starchy mush into smoother flavorless, starchy mush) but they have little culinary value left. At most they would be filling, but probably unpleasant to eat.

On the other hand, if you look outside of culinary applications, they make great compost.