Don't get the logic of dropping Hauritz for a fourth quick when as you've acknowledged here, there isn't really anyone who's demanding selection. George and Hazelwood are unproven rookies. Harris is injured/coming back from injury, and given the difference between him fully fit and only thereabouts, you'd want to see that he's recovered properly. Siddle is really the only one, and I don't know whether he's going to be any better than Hauritz in general. Whichever fourth specialist quick you bring in is going to be the fifth best quick in the team, after the current three and Watson, so what do we think they'll do that four better quicks won't get done? This isn't India that's about to land, and some of the Poms definitely don't know what they're doing with spin.
My reasoning is as follows:
1. I don't believe Hauritz has what it takes to be a legitimate threat against top order batsman (which is what would be required if he was to be in the side). If he is just in the team to to add variety and conserve runs etc. I think North, Clarke and Katich can do that job just fine (as the test series in India proved). Furthermore, given the good fast bowling line up (Johnson, Bollinger, Hilfenhaus) who will probably thrive in Gabba conditions, Hauritz could easily be targeted as the weak link.
2. I don't think Watson should be used much as a main bowler in this series. He is one of our best batsmen and should focus most of his attention of that job to boost our vunerable batting line-up. Therefore, if Watson isn't used much as bowler, a 4th fast bowler (not including Watson) would be useful to manage the load (especially as though Bollinger and Hilf have had injuries recently) and add more variety to the attack.
3. Seeing as though Johnson is completely erratic, it is always useful to have an extra option. If the bad Johnson decides to turn up, that leaves the side with only Bollinger and Hilf to rely on.
4. If the 4th fast bowler was a debutant, as I said before, that could potentially be a good weapon against England who know all about Hauritz, but would know little about the new bowler. I think Hauritz would be a source of comfort for England more than anything. I also disagree that the 4th bowler would be the 5th best in the team. Both Siddle and Harris look just as good as any of the current line up. And e.g. Copeland or Cameron could be better - who knows. You could argue that the 4th bowler might fail, and you'd have to rely on the other 3. However, the exact same argument could be made with Hauritz. There is nothing much to suggest he will succeed in Aus either.
All that being said though, it seems at this stage Hauritz playing is very likely (as per Chappell's comments). I just really don't like the look of the team with Hauritz in it though, especially if North and Hussey are retained. Because then the team is practically identical to the one that toured England - one England would know they can beat, and are highly familiar with. I feel this Ashes series should be the dawn of a new era for Australian cricket, not just a continuation of the same old mediocrity.