The usefulness heavily depends on the client/project this is meant for. What are there goals? What is there audience. And even then what might work for Site A might have the opposite effect for Site B.
Is this for a particular case? Or just selling the feature in new projects? You said you don't have their emails, but will you in the future? Or is there a policy that prevents it?
Since you can only perform these activities when tracking data, you are gathering data right? You mentioned event registration so I guess you get things like name and email?