My friend Glenn posts about free WiFi in the independent coffeeshops of Seattle, and points to a fantastic article on the subject in the Post Intelligencer. It's totally comprehensive--if you work on a laptop in a cafe, give it a read.
I'm proof of the phenomenon, actually--I was a very early WiFi adopter, using WiFi (before it was called WiFi) at Starbucks, back when that service was hosted by Mobilestar. Mobilestar died and is now T-Mobile, which I keep a monthly account with, because I was grandfathered in at a low monthly rate.
But I actually have only logged in with T-Mobile once in the last three months, even though I'm traveling--I'm using a free WiFi connection at Berkeley Espresso, because it has a cooler vibe, I'm sick of Starbucks and my wife and I can both be online at the same time...it's just better in all ways, even with the fact that since the pay service is subscribed to, I could just use that.
In the article the pay providers point to reliability as a selling point of paid networks, but in my experience it's mainly a hobgoblin--WiFi is so easy to configure and distribute that I have never actually had a problem with a home-grown facility, while on the other hand I've had many problems over the years with T-mobile/Mobilestar. It's simple, really--if Joe Coffeehouse's WiFi goes out, the folks complain that day and Joe fixes the router or calls the internet provider. If a T-mobile location goes wonky, *maybe* someone calls T-mobile, and T-mobile sends someone to look at the node at some point, in the future...you get the idea.
Long story short--I am keeping my T-mobile account for now, but after this latest trip I'm thinking about killing the account when I get home and paying as I go, given how infrequently I actually need it. Good news for me, and good news for WiFi--and bad news for the pay providers.