WiFi saturates quickly. Throw a few dozen users in the same airport lounge and watch as everybody throws their phones against the wall in frustration.

Handoff between hotspots is also pretty crappy. I can walk and talk from one end of the terminal to the other on 3G/4G. On WiFi if I'm not sitting still I'm going to lose my connection fairly soon.

WiFi really is only good for one thing in a public space context: someone sitting down.