A couple that I can think of off the top of my head...
Tomcat 6 implements the most recent Servlet and JSP specs.
If staying current with these is important, then you will want to upgrade.
Performance is always being examined by the Tomcat developers and in the past each new version of Tomcat has been faster.
If you're not current, you aren't getting the latest security patches (although the TC group is usually pretty good about updating older release versions when a security hole is patched.
Support: Most of the more knowledgeable people on the Tomcat users list are working with current versions of Tomcat and are better and more willing to help out people who are also using more current versions.