Criminal attacks are often opportunistic, and often all a system has to be is more secure than the next system. But there are other threats. Lawyers sometimes need a system attacked, in order to prove their client's innocence. Lawyers can collect details on the system through the discovery process, and then use considerable financial resources to hire experts and buy equipment. And they don't have to defeat the security of a system completely, just enough to convince a jury that the security is flawed.
Electronic vandalism is an increasingly serious problem.
Computer vandals have already graffitied the CIA's web page, mail-bombed Internet providers, and canceled thousands of newsgroup messages. And of course, vandals and thieves routinely break into networked computer systems. When security safeguards aren't adequate, trespassers run little risk of getting caught. They can attack a system using techniques the designers never thought of. Art thieves have burgled homes by cutting through the walls with a chain saw.