Watson gets out far too often by LBW to remain in the top 3. 32% of the time he is out LBW, in comparison Ponting 19%, Dravid 14% and Hayden 15%. Number 4 seems the best balanced spot for him in particular with his bowling.
And yet he averages mid 40s opening the batting? I think he's doing better in those LBWs now anyway. When he first started going LBW the concern was that he was getting so far forward that he had to play around his front pad. But that's not how he's been getting out LBW of late. Yes, Steyn got him LBW in SA, but it shouldn't have been out. Having a look back through his LBWs, last one before that (vs pace) was Bresnan at SCG where he let the ball go, one before that was Tremlett at WACA where he was 95 so I'm willing to forgive, Finn got him LBW with a yorker on the toe in the 1st innings. The last one I'd be concerned about was vs Asif at Headingly in 2010.
I'm actually more concerned with Watson vs spin. Yes we know he can hoick Bangladeshi left armers over wide mid wicket if he wants to be aggressive, but what about coming in at the start of his innings vs spin? I think he looks far more confident vs pace, so that's not a guy you throw back down the order blindly. Going to #3 wouldn't be so bad, but going down to #5 for example could be a problem, coming in against a softer ball and a spinner in their stride.
However that would mean Ponting has to shift back to the all important 3 slot. The problem isn't him in the spot, if he is deemed good enough to be in the team then he is good enough to bat 3 where he has for most of his career. The problem will come if he is moved along before the Ashes leaving us looking for a new number 3 again. With Hussey occupying the breeding spot we don't have a Ponting at 6 waiting to move to 3.
I think it's a bit rich claiming that #3 is super hard, but #6 is easy runs. I don't see it that way at all. #6 is less responsibility I suppose, but I think if Marsh is seen as the long term #3, then he's going to need to take that responsibility at some point. The question is where Watson and Marsh would bat AFTER Ponting and Hussey retire? After you answer that, then just put Ponting and Hussey in the gaps.
If your top 6 in 2 years time is something like this:
Cowan
Warner
Khawaja
Watson
Clarke
Marsh
I see 5 guys there who could open and all 6 could play top 3. Arguably Ponting should be going down to #5 above Hussey, so that the selectors can see which of these young guys can handle batting in the top 3-4. And I also see too many lefties. If Wade is the #7, that's 5 left handers. Graeme Swann would enjoy that.