Just thinking about my user-case for Pay with a Like plugin.
It would be great if there was an option for "After x pages" -- it'd be invisible for x pages, then start annoying people.
In fact, it'd be best if things could tie into the site's Facebook page, and ask people who
a) were signed into Facebook, AND
b) hadn't liked the site page
to give you a like after x pages.
A different user case, but definitely a valuable addition.