What he said. The ridgeline does maintain a consistant ammount of sag in your hammock, without one every hang will be slightly different causing the feeling and lay of your hammock to be different. This is what is known as a "Structrual ridgeline". Some people only use a thin piece of cord for a ridgeline simply to have a place to hang stuff and sometimes with the use of an additional bug net or sock the ridgeline helps to keep those things off of you while you sleep.
Beware of the man who owns one gun, he probably shoots it well.