There's been a lot of call lately for every Democrat to unite behind John Kerry as the Presidential nominee. This has some superficial appeal, but really isn't the best way to go.
The conventional wisdom is that a bruising primary with lots of mudslinging hurts a general election candidate, and this is true. Not only does getting slammed by people in your own Party weaken your support, but slamming them in return makes their supporters less likely to support you in the Fall.
But John Edwards has not run the kind of negative campaign that is actually going to hurt Kerry. Instead, Edwards has run a campaign based on the inequities in Bush's America and what he would do to redress them. This is exactly the kind of message the Democrats should want on the airwaves, and what they should want the voters thinking about. There is nothing in Edwards' message likely to hurt Kerry at all in a general election and quite a bit likely to help him.
The other reason Kerry needs Edwards in the race is that without viable opposition, Kerry stops being a story. Right now, John Kerry is a big deal wherever he goes and whatever he does. In fact, right now he's a bigger story than George Bush. Kerry can hog the limelight as long as he's in a competitive Primary. As soon as everyone else folds, Kerry's visits to swing states and policy speeches will get much less play.
There are plenty of Primaries left in swing states. It's in every Democrats interest that these Primaries get as much attention as possible and that the overall Democratic message gets out to the general public. They only do so as long as a viable opponent stays in the race, and John Edwards is the only one who hasn't gone negative on John Kerry.