Horses for courses. The 13" pro has more ports, can take 16 gigs of memory and has easily (read: apple sanctioned) user-replacable storage. it has a faster CPU.
If you don't need any of the above, are fine with 4gb max RAM, then sure, the Pro looks like a bit of a waste of time, but if those things are important to you (i.e., you're doing pro level stuff on it) the air is a non-starter.
Doesn't matter how light it is, and how fast the inbuilt SSD is, if you're trying to crunch > 4gb data sets on it.
Ditto if you need fast networking. Wireless N is reasonable with the theoretical 450 megabit you can get (300 on an air with only 2 antenna), but in the real world, it does not go through walls very well at that sort of speed, and the realistic max speed you'll see unless sitting on top of an AP is in the 100 megabit range. Shared between everyone on the AP. Vs Gigabit per user when using a pro on a wired network.
If you have multiple users in an office that all need fast networking, and/or you are storing and retrieving large data files from network storage, the air just won't cut it.
The air is a fine machine for light usage and portability (e.g., casual home user stuff, student usage, web development, etc, etc). It is NOT a Macbook Pro 13" replacement.