From what I can tell about ning from an administrative standpoint, it would only be useful if you have a specific use for it. Arlington Reads tried to set one up for its volunteers to be able to connect and get useful information. You can see that there are sort of spammer joiners, which isn't helpful.
From a user standpoint, I don't see what the point of it is, unless I went to a website that I really liked and their community pages were on ning. For instance, Texas Prairie Farms updates the people that buy their produce on what is going on.
I don't think I will use this beyond this Thing, especially with their terms of service.