I think you overestimate how many times a player has had to come in and save our asses at four or five. The list Beads provided is a pretty long one for a guy only playing his 66th test.
All of those though were at 5 and 6. He has never done it in the few matches he's batted 3 or 4. I can't say how many times he's been needed in those tests either, but the guy is a gun 5 or 6. He should be batting there.
Watson is a weird one. I really didn't think he could make it as a top order batsman, but he's making a good fist of it. Khawaja and Hughes are two guys who should soon be introduced into the top 4, and Smith shouldn't really be considered as anything higher than 6 I believe in tests. That to me says Watson may need to stay at the top or maybe move to 3. It's just the 4th top order batsman to join those three I can't put my finger on at this time.