a 15" display is only 27% larger in area (and thus backlight power) than a 13.3" screen. That is not a huge difference in power requirements. The overall case and battery size would increase at approximately the same rate so it is unlikely to be a real problem. Even the weight would only increase from 2.8 pounds to 3.5 pounds which is the same as the 14" MBP. That is still less than the 4 pounds of a 15" MacBook pro.An MBA is expected to be lightweight, and the low power CPU is only part of the power equation. With light weight and a 15" display sucking power, where is the huge battery life to come from? Apple has trained buyers to expect very good laptop displays, so I do not see where a cheap/light 15" MBA display is to get the huge battery life from.
Edit: Unless Apple has an inexpensive+good+available display tech we have not seen yet...
The point to a 15" MacBook Pro is that some people want a larger screen but don't need the weight, power, and cost of a full MacBook Pro. That is why that is a very popular size in the PC laptop world.
A lot of people aren't buying the MBA because it is lightweight, they just get it because it is the lowest price laptop from Apple. The next step up in Apple's lineup is a pretty big jump in price. A 15" would make them more likely to upgrade their purchase over a 13".