I think another reason we haven't seen a destructive worm yet is it would impede the further dissemination of worms.
Joe wormwriter thinks to himself- "I could make this devastating and bring down multiple thousands of systems. But how could I tell If I'm the best unless I leave a 'clean' field for others to show their wormwriting skills on?" That or how much money is expected from the spammer-patrons.
----------------------------------------- "If you don't vote, it's your fault!"