Open source software (like Linux, OpenOffice) allows for more innovation and development as anyone (including IT specialists within the government) can study the source code and improve on it.
Second, the government won’t be locked in to a particular vendor (e.g. keep paying out for Microsoft user licenses) and thus have more choices.
Thirdly, open source software has already matched (if not surpass) propietray software in terms of reliability.
In sum, the use of ‘open source’ software by the government would provide better value for money and would lower the cost of IT in the long run.