I discussed this with someone today. They used to do it.Fwiw, you can cut waste by feeding your feed moist rather than dry. Of course, it isn't pre-digested or full of probiotics, but they can't bill it out as easily as dry.
I haven't noticed a problem with flies. I usually feed just enough for one day, perhaps the birds gobble any slow, stupid flies before I notice.
I just can't see myself leaving the residue on the feeders from day to day.
My comment about the flies was in jest as much as any.
I was telling someone today. I am going to trial it three ways. A dry pen, wet pen, and fermented pen. Side by side. Related birds, same age, same pen arraignments, and in the same location. Same feed, just prepare differently. I will see for myself this year. I am curious to see if their is any measurable differences.