I think that the problem with this discussion is that people are trying to see things in absolutes. It is not logical to say that all yamadori are better than all prebonsai which are better than all nursery stock. I don't think anyone has claimed that. (Except Vance's statements that this is what people are telling him.)
But if you look at each group as a whole, the percentage of high quality trees coming out of each group probably does flow along those lines. If you took 100 collected trees from Oregon bonsai and 100 prebonsai from Brent and 100 nursery trees from your favorite garden/landscape nursery, you will have significantly more GOOD bonsai from the first 2 groups. That's not to say you can't get good bonsai from a nursery, but you have to look alot harder and alot longer.
I look at trees everytime I go to any nursery. In my 8 years of doing bonsai I have purchased I'm sure over 100 trees from regular nurseries to work on. Out of those, I think I have 4 or 5 that might someday make a nice tree.
In 1 day at oregon bonsai I bought 4 collected pines, all of which will make incredible bonsai, and they probably cost me about the same as those 100 trees from regular nurseries.
You can do bonsai any way you want, with any material you want. But shouldn't you start with the best material you can? Where-ever you can get it. Doesn't matter if you bought it from Wallmart or from Walter Pall. But start with the best stuff you can find/afford.
- bob