What is the practical difference between "closed wifi" and "open wifi with a mandatory log-in"? In both cases you must obtain a credential (and thus implied permission) to use the network. You've just moved the access limit from the radio to the wire side.
The practical difference is that someone can use it without having to knock on random people's doors to find someone willing to share the necessary key/credentials. It's an automated process. That's a huge functional difference. (Whether it's effective is a different question.)
In general, though, the reason this movement will fail is the same reason why people want it to work. Selfishness. The same person that says "I would like to have wifi without paying for it when I am somewhere not home" has already said "I don't want to pay for my own 3g/data plan so I can have network access when I am not home".
There's a very good alternative motivation, the same one that drives all sorts of stuff engineers, computer scientists, and natural scientists do: efficiency. We've got all these cables laid; why not use them? And we can maybe get better speed (and better reception indoors) in the process. Other practical questions aside, I'd like it if I could use other people's wifi/broadband connections and they could use mine because it would make more efficient use of existing infrastructure and cut down on congestion in the limited brands of spectrum allocated for such RF broadcasts.
I personally have little desire to freeload off anyone. I had actually even considered if there would be a way to setup some service where people could offer access to their wifi to other users of the service (essentially "I'll share my wifi if you share yours") and/or offer a mechanism to pay something to defray the broadband bill of the open wifi operators. The specific goal I had in mind was eliminating free riders and sharing costs along with access.
In the end I decided it probably wouldn't catch on if it were only for the initiated (others running open wifi), and payment would be too problematic because 1) it would be practically difficult to charge a reasonable fee due to flat transaction costs on things like credit cards and 2) it would probably end up with people spoofing access points to phish for credit card numbers. The point is, though, that not everyone who wants this stuff is interested in being a moocher.
It should also be said that companies offering wireless data service and wired broadband tend to be relatively uncompetitive cartels, so it open wifi allowed consumers as a group to effectively get a better deal from these companies (utilizing the bandwidth they pay the ISP for while avoiding unreasonable wireless data charges) that would be good too.