But eventually somebody will look and if they find something, they can just fork the code and remove anything malicious. Anyways, open source to me is not about security, but about the public "owning" the code. If code is public all can benefit from it and we don't have to redo every single crappy little program until the end of time but can instead just use what is out there.
Especially if we are talking about software payed for by taxes. That stuff has to be out in the open (with exception for some high security stuff - I don't expect them to open source the software used in a damn tank, a rocket or a fighter jet)