The SSO across all these apps is interesting. As its based on a 'global' identifier, the implication is that Ning would be able to correlate any one user's actions/identity across all these purportedly different applications.
It appears that Ning acknowledges this as a concern because the FAQ has the following
I want to use different email addresses and identities for different apps. Can I do that without creating two accounts?
Not at the moment. For now, you probably should just go ahead and use one of the many free email services that are happy to give you as many free email addresses as you want.