It creates all sorts of problems. Cabin crew then has to kick the visitor out at mealtimes. And how to enforce that the visitor doesn't start "tasting" the food and end up eating enough for a meal. Also, QR, as much as or more than other airlines, tries hard to treat its F passengers very well. Part of that is keeping the cabin from becoming crowded and maintaining the open, calm and quiet feeling that it has. Count me as one who doesn't believe that visitors should be able to visit "up" cabins. But the person in F can always visit others in Y and J, passengers in J can always visit others in Y.