I think her calculations are a bit more subtle. First, I think she was going to resign/retire as SoS even if Obama won, so she isn't giving up much. Second, if she hides on Benghazi, it will come up in 2016. I think she thinks she's better off hoping her cooperation helps him win a 2nd term, and then she runs in 2016, rather than trying to run against a sitting Republican president.
Third terms for one political party in the presidency are historically very rare.
The clintons want no part of Obama being re-elected. Hillary’s chances at the presidency are about 100000 times greater if she is challenging Romney for re-election.