It's unfortunate McNealy failed to immediately raise the obvious benefit of transparency. Open source allows the government and its citizens to more easily verify that mechanisms follow policy and law. This is particularly important for things like voting, surveying, validation and compliance testing where even small faults can cause serious problems and/or compromise the security of the system (e.g. http://www.mail-archive.com/hydro@topica.com/msg00406.html )
David A. Wheeler has a good collection of other reasons here as well: http://www.dwheeler.com/oss_fs_why.html