-Limiting exposure of the information already divulged
-Preventing further (especially unintentional) classified information spills
-Limiting adversaries collecting on what information and to what degree people are knowledgeable about
The fact is, if you're a legislator who is given compartmented access, you should know better than to propagate information that has been unintentionally released. For one, it promotes discussion of classified information in unclassified spaces. Secondly, it gives apparent validation to information that might not necessarily be true.
It's getting really tiresome reading these flavor-of-the-day NSA bashing articles, having worked at NSA and having eaten of the tree of knowledge. The intelligence community does not dictate foreign policy or collection policies. They serve at the direction of the federal government under the DNI (http://en.wikipedia.org/wiki/Director_of_National_Intelligen...), who is appointed by the president. They carry out the tasks that are given to them under the direction of the administration. There exists a feedback system which allows agencies to understand what information was vital in helping make decisions and what information wasn't important.
Here comes the tricky part: politicians clamor for more information in order to make better informed decisions -- just look at the situation with the possible use of chemical weapons in Syria. When intelligence agencies don't come up with anything actionable, politicians ask "Why not?", to which the agencies respond, "We don't have enough access to real time information.", or "You stated that X mission wasn't important so we stopped supporting it.". Then the politicians rewrite legislation in order to prevent further roadblocks to obtaining information in a timely manner with which they can make decisions.
All of that happens, and you want to blame an intelligence agency for doing its job at various levels of efficiency? In that entire process, the agency only gives feedback as to what its capabilities are and what it needs in order to increase its capabilities. The administration and legislators determine the scope of the work which the agencies function in.
There is no secret conspiracy, people. It's a very simple system: you vote in people and then tell them that you want the prevention of terrorist attacks prioritized above funding NASA. The public is then made aware to what degree the politicians were willing to go to in order to expand intelligence powers, so outrage ensued. Now, there is legislation being pushed forward (not successfully yet) in order to limit the powers. Is this not a representative democracy working to correct its mistakes and push towards a more perfect union?