This is true. What may also be true is that the next time you upgrade your MBP (3 years maybe?) would you need to upgrade your TB display? Or could your display be used over the life of 2 MBP's, substantially reducing the cost of your next MBA-as-a-desktop.
Good point since my iMac has a second display (a 20" Dell 1600x1200) which is now about 10 years old. But, you know, the *next* TBD will be Retina, and I'll just have to buy it.