Willard, I agree.  I was more thinking that if someone wanted to do milling and felt that they weren't confident with hand sharpening, the mechanical contraptions offer something.  Chances are at some point they would move to the more convenient hand method, that seems like the natural evolution.   If you are regularly hitting something like some marker plates that were in some orchard trees that I had access to, in every tree, I think that a sharpening device could be useful.  i didn't have one with me, and I learned that even by hand, you can almost use up a brand new chain in a day.  it was terrible, so frustrating.