Even if your site does not contain any valuable information like credit-card numbers, it is still a target. It has an IP address, which is all that worms, crackers, and spammers want. Worms in particular are quite indiscriminate about who they attack. It's a rare website indeed that can go more than a few minutes between worm probes.
You must expect to be attacked. More, you must expect to be successfully attacked from time to time, and plan accordingly.