Including links to other sites raises a similar dilemma: Do you offer the visitor an easy opportunity to leave you for your competitors?
If your "competitor" is really offering content which is different than yours, you could help each other by cross linking to each other's web sites. On the other hand, if you and your competitor offer basically the same information, it would be foolish of you to point your visitors to the other site. The answer then depends on whether you are running a "commodity" site or one which is unique in some important way.
If your clientele is made up mostly of utilitarian searchers (see Chapter 15) and the reliability of the information is an issue for both you and your competitors, it can make sense to point to other sites since you are helping the visitor by giving them a larger sample to use to make a judgement. If reliability is not an issue or is actually very high, so that all sites have the same information, then it would not make sense to link to competitors.