I personally would never post a link to my site (or to anybody's site) if it was not relevant to the subject and if I hadn't the belief that it is beneficial to the reader.
While that is a good start, it's clearly not enough to ensure that a link is valuable. I'm sure all the people who post links to their collected sermons of their favorite religious sect feel that their links are relevant to the subject and highly beneficial to the reader as well.
Jay.