The tc5-cluster.sar deals with web session replication. The deploy/cluster-service.xml file (not "cluster-config.xml") allows other clustered services, i.e. clustered EJBs, HA-JNDI, farming, HASingleton (needed for HA-JMS), etc.
Have a look at http://wiki.jboss.org/wiki/Wiki.jsp?page=JBossASTuningSliming -- near the bottom there is stuff about removing clustering services that you don't want.
If all I want is Session replication, I need the tc5-cluster.sar application. The guide you pointed to is an all-or-nothing removal of clustering.
It is my understanding that underneath the hood, the tc5 clustering uses JBoss Cache. I'm not sure how everything's plumbed together, so I'm just going to delete cluster-service.xml and hope for the best...
Alternately, could I get http session replication by starting with the default config and copying tc5-cluster.sar in from the all directory?
You'd need to copy jboss-cache.jar and jgroups.jar from server/all/lib as well. I think all other jars those two depend on are already there in default.