If Edwards drops out earlier, even if he endorses Obama, a good chunk of his supporters will be inclined to go with Clinton. After all, they largely fit the same profile as Hillary's supporters (white, lower-middle class blue-collar working families). Besides, let's be honest, some of them are probably racists. Others will be women who had not previously considered the choice between a woman and a black man and who will now go with the woman. Others will decide that they need a fighter, not a uniter, for their interests. That fits right in with Hillary's message. Even a passionate appeal from Edwards himself will only leave a lot of his supporters vulnerable to poaching by the Clinton camp.
Now imagine that Edwards endorses Obama with hundreds of Feb. 5th delegates in his hip pocket. Even with all of the different delegate selection processes in the Feb. 5th states, I have to imagine that his campaign would have an easier time of ensuring that his delegates go to Obama. In Arizona, for example, he could allow Obama's campaign to recruit his supporters to run for Edwards delegate slots and veto any known Clinton supporters. It would require a lot of effort on the part of the Obama campaign, but this is a delegate fight, after all.