I don't understand.
The air is faster, more reliable and lighter. It has a better battery life and a higher resolution display. External storage is cheap, and external cd drives do the job just fine. There are very few valid reasons to buy a 13" MBP
Mac Pro has higher performance, and is user upgradable. Some people want/need DVD-drives too, and don't want to mess with external crap.
Edit: They also has more ports (Ethernet, Audio In, FireWire).