If the Bush administration had launched an attack on Clinton policies, it would have been easily dismissed as partisan politics, decried as unfair by the mainstream press, and immediately ignored.
By letting the Dems light the torches and collect their rudimentary farm implements, this has given them the feeling (those are important, feelings) that they were going to hang Bush out to dry before the election. They even stacked the panel.
By giving the Dems (and the media) all that rope, and letting things gain momentum, the media can't just pull the plug. By dropping the bombshell of the Gorelick memo, and the rest yet to come while things are rolling, more people are paying attention who might not have been otherwise.
There is the added benefit that the Dems wanted this, stacked the panel, and now it has backfired. This is a Texas prizefighter defending himself in a bar brawl. They took the first swing. IMHO a lot of worms will be out of the woodwork before this is over.