If they launch an investigation, the investigation is likely to entail Rove's email. Granted, it can't be a fishing expedition, but they might be able to gain some insights on the 2004 "strategery".
Since they can't beat Bush with their candidates AND they can't really beat Bush with the lying and deceit that is the demoCREEPs typical tactics, they need to see the "playbook".
That, or the demoCREEPs are trying to pitch a red herring to cover up the fact that a $50,000 donor to Hillary was just arrested with known financial connections to the terrorist group Hamas.