Granted, the notion of checks & balances is an inherent part of our founding structure. If one unit of government might overstep their bounds, having another unit incentivized in opposite direction (but both having reasonably defensible mandates) is fine.
Government moving slowly through bureaucracy is one level of protection against individuals usurping the system and establishing their own agenda. (though there are many ways they still do this)
The NSA itself contains diametrically opposed missions. It was easier when the signals and infrastructure of "us" and "them" were mostly physically separated. Bruce Schneier has suggested breaking it up [1].
