I'd get some clarification before just calling him an idiot. He very well could be one but I don't know for sure yet.
I don't mix anything into my burger either, but when I make summer sausage I do, because the recipe calls for it.
As to mixing beef or pork fat with burger I don't think it's a good idea at all. Fat is the first thing to taint in the freezer, and it'll take the meat with it. Straight ground can last several years in bags that are sealed tight with little to no air in them, if you mix fat with it you've got a year, two tops usually. If it's kept below zero it's safe to eat indefinitely, but that fat will wreck the flavor long term.
Straight ground meat is fine for pasta sauce, tacos etc.
If I want to make meat loaf, burgers etc. Then I add just a little pork breakfast sausage to make it stick together and make it juicy. Perhaps that's what the other guy does too.
