Comparison is flawed.
If you need more computing power than the MBA, then you can select the trim of Macbook that meets your needs.
Granted, the first tier of quad-core MB is the 15" Pro, which is double the price of the Air ($999 vs $1999). But the point is you can still get a Macbook with quad-core if you want, even if it means picking up a bunch of extra things along the way (16 vs 4 GB RAM, retina display, 256 vs 128 GB storage).
For a headless Mac, you have no choice but to spend a huge amount more on a Mac Pro.