If the progressives and liberals bolt from Obama as they surely will if Dean challenges Obama, that leaves Obama with the Blue Dogs in the Democratic primaries in 2012. The progressives and liberals control the path to the Democratic party nomination, that's why Obama garbed himself in progressive clothing in 2008 but his stealth candidacy has now exposed him for what he is: a DLCer. Obama doesn't have the warmth of Dean or Clinton to overcome his DLC positions and win the nomination.
Moreover, Dean can use the "change mantra" on Obama in 2012. Wouldn't it be a pleasure to see Dean taking the 2008 Obama position and Obama taking the W role in 2012?