My best guess is that they expect to lose the Senate in 2014 so they see this as their last chance to stock the courts with ideologically left judges.
The Senate is different. There is a good chance that the Democrats will lose control of Congress in a year and the White House in 3 years. There will be lots of opportunities for them to regret setting this precedent. So why did they?
Obama's power grabs are easy to understand. He has little respect for the Constitution and hates working with Congress so he simply goes around it. He will not have to live with the consequences since he will be out of office before anyone else can follow his precedent.
Last week the Senate joined the President in grabbing power. They exercised the "nuclear option" and eliminated the filibuster for certain offices.
Remember back when President Bush was a threat to the Constitutional fabric of our government because of the number of signing statements he made? Consider our current administration.The President has expanded the powers of the office in many significant ways. In some case he has ordered agencies to assume additional power as the EPA has by bypassing Congress and ordering new mileage standards. More worrisome, he has simply used executive orders to bypass laws he finds inconvenient. The most significant of these were his order implementing the Dream Act and his postponing enforcement of elements of Obamacare. Bizarrely, he threatened to veto any legislation that actually gave him the authority for these actions.