I don't really see why Watson has to make the team as a batsman alone. I'd agree that in test cricket he'd need to justify his place as a batsman, but in ODIs that's really not the case at all, and there's plenty of handy ODI cricketers out there who make the side based on their all-round ability and not skills in a single discipline.
Watson's valuable to the ODI team because he offers balance. With no Watson, you either pick a seventh batsman and look for 10 overs from Symonds and the part-timers, which reduces the flexibility of the attack as well as the overall quality, or you pick a fifth bowler or a different all-rounder. With Watson in the side, the bowling attack at full strength has more variety and there's a better balance to the team in general, with seven batsmen plus your Hogg and Lee types and five or six genuine bowling options.
Once it's been established that Watson is of value to the side, the batting order should be determined based on the strengths of the players. If Watson was a mediocre batsmen in all positions, there'd be no justification for batting him up the order, but the fact is that he's done very well from limited opportunities up the order. He's now got five of his six half-centuries in the top four, and he's played under a third of his innings there. Top order average is in the mid 40s, and his SR is better than most as well, in the mid 80s. There's plenty of better finishers in the team, and Watson's always done quite well at the top of the order. There's no real reason not to bat him up there, and move quality finishers like Clarke and Hussey further down.