If you are a big government, M$ will show you the code. This has some consequences:
- some governments may learn how to hack into the monoculture of PCs globally for espionage or censorship
- other customers of M$ may realize they are second-class customers of M$
- this may promote/certify FLOSS as valid, undoing years of FUD
- governments that see the code may be horrified and ban that other OS in their jurisdiction
The idea that there is security through obscurity disappears when you know some really bad dudes have access to the code. Many governments have really bad reputations for lack of respect for privacy and their own laws. Giving them access to the code while denying it to other customers may not be in the best interest of others.
I would rather use software where everyone is on equal footing, FLOSS, such as Debian GNU/Linux