'Sharpening' means different things to different people. For some guys, it is a few swipes with a sharp file to maintain a sharp edge. They do this every tank of fuel or so. For other guys, it is when the back of the cutter cuts as well as the front. They get 2 to 3 'sharpenings' out of a chain. Some guys never remove the chain from their saw until they buy a new one to replace it.
I sharpen batches of chains that have been used by storm cleanup volunteers - they hit a lot of debris in that stuff. I also scrounge, salvage, and save chains. I really like my 511A for that. I also like to carry 3 chains for each saw and swap them out in the woods (might do a light file touch up) - gets more even wear on the chains and sprocket.
Everybody has to find something that works for them. Mattfr 12 has decided that files work for him, so it is a win-win situation to offer his grinder to others. Looks like a good deal - if I was not already 'grindered-up' I would take a closer look!
Philbert