No team has ever used a 6-man rotation for an extended stretch. First, your best starters should pitch every 5th day, whether you have 1, 2, or 5. Second, many pitchers actually pitch less well with too much rest. Third, it means the bullpen is one man smaller. Going with it on a short-term occasional basis or using a one-time spot starter is probably beneficial in special circumstances but I think it's bad strategy generally.