As a consumer, I'll engage with popovers if it's content or a publisher I'm especially interested in reading more about. An example of this might be - let's say I already donate to or am active with an organization, and they have a static newsletter signup link in their site's main nav... I'll never click this, but I will engage if they prompt me.
Jordan Coeyman's OptKit is a light but intelligent popover tool that actually works, and I signed up for his newsletter based on how well the tool worked on his blog.