The harmonic balancer is the crankshaft pulley. It consists of an inter metal pulley, an outer metal ring with a rubber dampning ring between the two. This is to reduce vibrations as you concluded.
The eariler models with the V-6 engine (1998 and 1999?) were equipped with factory defective rubber dampning rings. The rubber ring would split causing the harmonic balancer to seperate. Since the failure occured after the factory warrantee expired the expense was the owner's. Mercedes has since modified the harmonic balancer to prevent premature failures.
The only way to prevent it from failing is to replace it before it fails.
If you contact your local Mercedes dealer with your model and vin #, they may be able to tell you if your harmonic balancer is equipped with the modified harmonic balancer.
1998 Mercedes E320, 200K Miles
2001 Acura 3.2TL, 178K Miles
1992 Chevy Astro, 205K Miles