I think a question to be asked is:
As most of us practice bonsai as a hobby, we have to establish what bonsai means to us...is bonsai about having world class kokufo ten trees, or is it about the journey to creating such a tree ?
That being asked, I think its al relative to personal perspective...hobbyists generaly practice their hobby because they like doing it, aka the journey... I like working on my trees as much as I like viewing my best trees, even if the material is bad,mediocre or awesome...
Masters will not bother with bad material because, bonsai is their livelyhood, their bread and butter. Its just not worth spending the extra time on bad material so they could create something good out of it, they will lose a lot of money...that time could be better spent on two or three trees of good to exceptional quality.
Lastly, coming back to the journey...I know the topic is if a master could create good bonsai from bad material or not...but jeez guys, why should all of our trees be world class...why not just love to work on our trees instead of rushing to be competitive with japan or europe
Anyhow...just the ravings of a South African