Worse than you might think (Score 4)

It's worse than just trying to fight off skilled crackers, etc. During a brief stint at Los Alamos as a researcher I heard this story: The classified portions of an MS Word document were highlighted and cut out so that the document could be sent to individuals without the proper clearance. Unfortunately, because the "Undo" feature works across sessions (the undo information is stored in the saved document) all the uncleared recipients had to do was Edit->Undo to see the classified portions.

The lab could educate the secretaries and researchers about the "gotchas" of every commercial product they use (and they do try), but people are bound to forget or make mistakes. If they deployed open source software they could inspect and modify the code to make these holes unavailable.