There have been a few posts suggesting that people should just use their carrier's data plan. But remember, WDW is an international destination with visitors from all over the world.
Using my own carrier's data service for my smartphone in the US is completely outrageous. The roaming charges are insane. When I visit WDW I set my phone to "Airplane Mode" (ie: No phone, text or web service).
That being said, having wifi available would be a nice feature, but, I wouldn't pay extra for it. I just can't see myself paying extra to surf my phone (and for the added distraction) while in the parks. I would much prefer to have a cheaper option in the resorts. $10/day is too much, especially when there are off-site locations offering WiFi for free.