Don't most working large-scale WiFi installations intentionally limit range? More computers talking to one AP is cheaper, but also doesn't work at all. So the question is: how much money does one want to spend on access points (and their locations)? If the answer is a lot, 5GHz makes a lot of sense.

(Also, how much does open air attenuate 5GHz signals, anyway?)

Edit: Interesting reading: http://www.qsl.net/kb9mwr/projects/wireless/pwr.html