The thing with the MBA is that it has an SSD as standard. Using a swapfile on a regular HDD is bad because it wears out the disk, increases the probability of disk failure and it's slow. With an SSD you have none of these issues, thus making 2 GB of RAM on an Air less of a "limitation" than 4 GB of RAM on a Pro (given that the Pro has a regular HDD).
I mean, of course no page outs is better than having to use a swap file, but with an SSD there's no need to worry about it.
SSDs are still a lot slower than actual RAM.