It's actually quite an interesting argument...
Is the expansion necessary for the development of the South East and UK economy, or is it required to provide the ability for airlines such as BA, Virgin, BMI and the various alliances based at Heathor to feed each other more traffic. The statistics on the number of people who get off at Heathrow or transit to another UK destination would be useful as that would lend weight (or not) to Ayling's argument.
I've no doubt that the argument is a complex one, and I'd certainly like to see the airports in the South East developed or replaced and a more reliable and useful infrastructure put in place (in fact this might be the best use of public money in terms of investment in the next 10-20 years). I would argue that it's better to ensure that what we've got is used and maintained better than it currently is whilst planning for the future.
I can't help thinking that Dubai (and Hong Kong and a few others) have all had the right idea in building their newer airports in new locations. Sure, the environmental impact needs to be detremined and analysed, but if the answer is that we need another runway, but perhaps Heathrow is not the place to put it?
(Here we go again!)