Cooking the onions with the beef saves some cooking time, though it does drain off a little of the flavour along with the grease. I often prefer the time savings compared to the small drop in flavour. Another flavour consideration is the fat content of the beef; the more fat in the beef the more flavour, and the more grease to drain off. Medium ground beef will have more flavour, and more grease to drain off. Using lean or extra lean ground beef will result in very little grease to drain off, and a little less flavour.