I used to roast on a 2 kilo ambex at a shop I used to work for. We also picked, but we didn't buy the best green back then. Now I really don't need to.
Hope that helps. Picking defects sucks.
As a tech, I definitely support the use of beans with fewer defects, especially foreign material, as well as robust visual inspection and magnets. Though most debris just makes lots of noise and possibly smoke, it can shut your customer down until the debris is removed (which in many cases, involves a tech). Most are pretty grumpy with their supplier when they find out what happened.
I've seen some pretty crazy debris in customer grinders, including some very large rocks (the largest was about the same diameter as a quarter). I've also had to replace burrs, which can be pretty pricey in the case of bulk and conical grinders.
I typically try to smooth things over and explain that coffee is a natural product, but this doesn't always work. This is easier when the debris is small and non-ferrous - missing a piece of large and/or shiny debris or something that a well-placed magnet would have caught gives your customer the impression that you aren't paying attention.
No, this doesn't answer your original question, but this is a side of the issue that I don't feel gets much attention. Figured I'd mention it as another consideration for why visual inspection and buying good greens are important.