After watching the video on installation I see there is not only a serial number validation required, but also a domain name required. So, how often does the plugin require a check with your server and what happens if your server goes down? Does the plugin refuse to work?
I ask because I have had a bad experience with this type of thing; the registration server went down when the company called it quits and I was left with worthless software that I had wrapped quite a bit of work around.