I guess that's true if you end up with a lot of trimmings. I was thinking of storebought, which... actually, could be made from scraps and other things that wouldn't otherwise be used.

In my own situation, I just wouldn't have enough vegetable trimmings to make stock -- I use the whole vegetable when I cook! For example, I never cut carrot tips off, and I only cut off the black part of the top, which is tiny and wouldn't really go in stock anyhow. Same kind of situation for celery, onions, peppers...