Although John Edwards is quickly losing his status as one of the front runners in the campaign, he is still vital in keeping Hillary Clinton from winning, according to a recent article on Alternet.org. Although a lot of Edwards supporters would likely switch their votes to Obama, he is still important for this reason: Obama still keeps most of the southern Black vote, but Edwards splits the southern White vote (in which Obama is trailing) with Clinton, preventing her from breaking even with him. Were Edwards to drop out, Clinton would quickly be even with- or even surpass- Obama in states where he currently has a lead.
This article says that Obama currently isn't strong enough, especially among many White voters, to hold his own against Clinton in the election, but if he stays strong and has a good showing on Super-Duper Tuesday, he should have a decent chance of getting the Democratic nomination. But up until then, Edwards is a vital part of ensuring his success.