The only reason that comes to mind when I'm told that some "butchers"
put in fat is because they normally would throw it away,  but can make money by putting it in someone's venison and charging fo it.
    The reason that all the recipes you've found say to add pork butt and good beef is that it works, creates the best flavor, creates a mix that holds together, and uses quality meat.
    I have never advocated or used fat or suet in ground venison, and refuse to do so if a customer asks. If they insist then we recommend they take if to a  "butcher."

Dave Firnett
Meat Merchandiser