In the MacBook Air, the SSD chips are soldered to the logic board. It is not like there is a choice on what kind of drive can be installed. When 64GB isn't enough, there is no way to upgrade. When the SSD gets a fault, there is no drive to swap out - it would be time for a new logic board. With NAND Flash having a finite lifetime, soldering the SSDs to the logic board is a prime example of planned obsolescence. When the SSD dies (when, not if), there is only Apple to turn to, so Apple effectively has vendor lock-in as well, but we have come to expect that from Apple.
No, the SSD's are on a removable board. See http://www.ifixit.com/Teardown/MacBook-Air-11-Inch-Model-A1370-Teardown/3745/1 (It's the thing that comes off from above the RAM)
Any given program will expand to fill available memory.