One of the use cases for RULES are updateable views (although that changes in 9.1 as that version introduces INSTEAD OF triggers for views)
Another good explanation can be found in the manual:
For the things that can be implemented by both, which is best depends on the usage of the database. A trigger is fired for any affected row once. A rule manipulates the query or generates an additional query. So if many rows are affected in one statement, a rule issuing one extra command is likely to be faster than a trigger that is called for every single row and must execute its operations many times. However, the trigger approach is conceptually far simpler than the rule approach, and is easier for novices to get right.
(Taken from: http://www.postgresql.org/docs/current/static/rules-triggers.html)