You know who is going to buy accounts on metro-scale Wi-Fi networks and not even feel the pinch? Those with high-speed DSL, cable, and dedicated line connections already: I haven't seen this issue discussed one whit. Everyone keeps writing and talking about Wi-Fi networks that span cities and towns as primarily serving an audience that has no connection, has a high need of mobile access, or that has dial up and wants to move to broadband without paying a regular rate of $40 to $60 per month. (Let's ignore those promotional $15/month rates unless they really become regular--not for the first 6 or 12 month.)
But here's who is going to be among the other large consumer of $20-odd/month unlimited mobile/fixed Wi-Fi: People for whom Internet access is like breathing a nitrogen-oxygen mix. If you're part of the crowd that spends above average on broadband, say $60 to $100 per month for higher speeds or special features, you probably also own a laptop. Which means that an extra $20 per month for roaming VoIP, Skype, instant messaging, and all the non-business aspects won't be a big deal.
If you're a business owner--home, small, medium, or large--$20 per month as a backup policy against a broadband outage or a line cut that would take down a wired service is a pretty low price to pay just to have it immediately available as needed.
Remember that many of the RFPs issued by municipalities require net neutrality to be enshrined in proposals. Which, in most cases I've read, includes an explicit mention that any device may be attached to the network and used for any legal purpose. Thus sharing a single network connection when a business's wired line goes down is perfectly legitimate.
The municipal architecture for most cities is either switched or mesh throughout, and it's only dependent on a supply of power--I don't know city-by-city requirements for backup power on mesh nodes, and I think there's essentially no requirement for this. In Tempe, I believe six fiber drops serve the MobilePro network, with at least one dedicated to city purposes. Because they're switched, even multiple fiber cuts wouldn't damage the network. Likewise, a network like Philadelphia's, according to EarthLink's description, will be almost entirely wireless until you hit some fiber points of presence.
This alternate infrastructure could become extremely popular. Which then begs the question: When these networks are actually operational and where the operators are obligated to or desire to resell access on a wholesale level, will Verizon, AT&T, Comcast, and others come around and simply make this a checkbox on customers' bills? "Add Wireless Philadelphia for just $17 per month! A discount of $5 per month off retail!" I'm guessing yes, because it will become another tool to reduce churn among its best members, and the equivalent of the limited free dial-up access that broadband providers offered in the early days (some still do).
Metro-scale Wi-Fi as a wired backup? It's a concept.