In an effort to reduce vulnerability exposure the US Army is adding Mac OS X into the mix of possible targets. There's nothing wrong with this approach. These days different organizations apply various methods to reduce the risk of incidents.
In my younger and innocent days I was under the impression that the government utilized custom applications running on custom operating systems designed by them for them. I guess the government doesn't have enough budget and resources to maintain teams of engineers and support staff to design and implement custom information technology infrastructure.
Most think and will say that this is a complicated issue with many pros and cons. However, if one really thinks about it then it's not that complicated. Investment into custom code will outweigh all the cons in the long term. Look at all the recent reports about cyber warfare attacks and their success. It was largely due to known vulnerabilities in the common software products.