I'm a software engineer and I find Wi-Fi in the air really useful. I hate early morning flights and redeyes, so my options for traveling eastbound across the continent are pretty limited. I usually end up leaving around noon or 1pm, and I feel obliged to take the whole day off work. However, when the flight has Wi-Fi, I don't feel bad about working for a couple hours in the morning and then the taking rest of the day "working" from 35k feet.

So, Wi-Fi on a flight saves me tons of money, since it means I save a day of vacation. Also, it means I can be a bit more flexible in picking cheaper flights and the savings more than pay for the wi-fi access.