OK, yes - heterogeneous systems are good. In fact, I nearly gave an example that with a, usually Windows, MS-SQL server sitting behind a hardware firewall, which is often Cisco but could be anything. The fact that it's not worth an attacker hacking the firewall device because it's too hard and not worth the effort does kind of imply that having different systems is a good thing. It doesn't help protect the open route but it does help protected the firewall 🙂 In a big IT shop IT staff should be encouraged to have a mixture of systems, partly for support reasons (hard to support a system you have no access to) but also because an attack against one shouldn't affect the others.