I wonder if her not testifying is a precautionary measure in case she decides to run for President next election. Wouldn't want anything on tape or on the record that might look bad. Benghazi was a mess up and she'd probably like to keep as far from it as possible.
The smartest thing that she could do now is resign and throw the blame at Obama. He's been safely reelected, so he won't have any trouble with an impeachment (as if Harry Reid would ever allow it on the senate floor), and she can claim that she wanted to act, but was constrained by the White House. The rumor mill already credits her with supporting Panetta and Gates against the White House in the Bin Laden mission (apparently, Valerie Jarrett was opposed to it, and had Obama put up all manner of obstacles). This would burnish her credentials on foreign policy and make her appear tougher than she really is, while weakening Obama. That would set her up nicely for 2016. Without that, she's the architect of his failed foreign policy, and the moment she's out the door, he's going to start blaming her for everything that goes wrong in foreign policy over the next couple of years (and things are going wrong at a rate not seen since the Carter years). She can avoid being slowly ground under the bus by getting in a preemptive strike.