Bruce Schneier mostly runs Windows. The NSA uses several different versions of Windows and many different flavors of Unix and Linux. I'm sure they have BSD boxes somewhere in their massive inventory, but it is by no means their primary or secondary computing platform. Why do you suppose that is?
It's because computer security is only a small piece of the security big picture. It doesn't matter how technically secure your systems are if you have a malicious trusted insider carrying sensitive data out, or performing sabotage. How resistant is your entire system to rubber hose cryptanalysis? If a bunch of guys tried to forcibly take control of your data center with machine guns, how secure would your system be? The NSA has offices out there with a bunch of Windows XP boxes - but where they have customized hardware based encryption with at the data entry/exit points, incredibly strict key management policies, TEMPEST shielding, armed users, detailed destruction procedures, and incendiary grenades sitting in the corner.