In my eyes the value proposition is that people can replace their usual URL shortener with shrtn, and potentially earn money without having to sign up to multiple affiliate schemes etc.
If the market of people who are bigger than JoeNobody but smaller than BigTimeBlogger is big enough, then this is a service that will likely fill the market and do very well.
Services that automatically generated shortners for user-submitted links (e.g. twitter clients and discussion forums) are also logical targets for such a service.