They have to draw the line somewhere or else the limit of 4 isn't really a limit at all and the issue of large groups of people using DAS as a "Disney hack" doesn't improve at all. There's no 100% foolproof way to prevent every member of the party from using DAS as a way to join 2 lines at once, but limiting the party size and restricting what exceptions are granted will curtail some of it. If a group of 6 friends want to ride 7DMT when there's a 2-hour wait, then at least 2 of them will have to physically wait on that line now, as opposed to the entire party using those 2 hours to ride Haunted Mansion, Big Thunder, and Little Mermaid (for example). Even if the other 4 do that, it's still 2 fewer people in the other 3 lines. Multiply that by the number of large groups where someone has DAS and it should improve wait times throughout the park, particularly Magic Kingdom.