Whilst this is a sensible precaution it is a poor basis for long term security. Information tends to leak out, and attackers are often skilled at eliciting information from staff using social engineering techniques. Many organisations assume that an attacker won't be interested in them, and that they are therefore unlikely to be the target of an attack. The rationale behind this stance assumes that a site is targeted because the attacker is interested in the information stored on it. Some attackers regard themselves as electronic freedom fighters battling against the commercialisation of the Internet, others wish to sell the information they glean, others are motivated by the power and control they wield over the lives of the system administrators they affect, and many attacks are motivated by revenge.
Attacks generally involve several computers and a multitude of accounts.
An attacker may capture accounts and gain unauthorised access to several systems before reaching his real target. A site can be compromised for no other reason than to provide a staging post for attacks on other sites, and to the attacker, it means little more than another IP address. Simplicity Software is complex. As the size of a piece of software grows it becomes increasingly difficult to test all eventualities. Complex code will probably have unknown loopholes that an attacker can exploit.