Appears that your question has been answered, but I'll just add another scenario where this will be nice. Often times in the waiting rooms at hospitals and doctor's offices there will be a strong WiFi signal, but no internet service on it and to use my device I have to manually disable WiFi to force 3G. I don't have the beta installed, but it looks like this feature will automatically do the switching.
Let's hope that it takes care of that. I also have 2 scenarios where this would come in handy.
First, at my work we have this guest WiFi network. There is a security policy on it that requires you to open a web browser and re-acknowledge the disclaimer after you've been idle for some time. The funny thing is that when this is being "forced" on you, your iDevice remains connected to WiFi and still has a valid IP; however, whatever apps are trying to connect to the Internet are SOL until you open up Safari, try to browse somewhere and re-acknowledge the disclaimer. If this feature works as desired and can automatically determine that despite the fact that WiFi has valid IP, it is useless and switch over to cell data, that would be huge.
Second, my cable company has WiFi hot spots throughout the area. In some cases they work, in others they don't despite the fact that the iDevice appears to be connected. Again, if this can determine that WiFi is useless and go to cell instead, it would be nice.