The Merc has a real general story about Wi-Fi that looks briefly at the free vs. fee topic: The author concludes that free will be the norm in the future, but he somehow draws that conclusion by making the point that today most users have their service paid for them by their companies. I don't think that's an argument for why Wi-Fi will ultimately be free. It could turn out to be like cellular where some users have their service paid for by companies but others are willing to dig into their own pockets.
One analyst makes a good point in the piece though. He says that until more of the operators sign roaming agreements with each other instead of promoting their exclusive networks the market will remain chaotic. It seems like the market is moving in that direction and roaming should open up the market.