i would have thought sensible strategy at this point would have been for all the remaining candidates to through their suport (and their support bases) heavily behind Kerry
I mean, the democrat goal is a democrat president right?
They probably will shortly. There are a few things that are still up for grabs in this race though.
First off, thre's the Vice-Presidency. It's not exactly a plum, but it will give whoever gets it the clout to have some say in the appointments of any cabinet or high level officials. It will also set them up to be a better Presidential candidate next time. That something that a guy as relatively young as Edwards could benefit from. After all, look at Nixon, he was Eisenhower's VP and got himself elected after a break of two Democratic presidents. To get the VP role, you either have to be liked by the Presidential candidate's supporters or you have to bring your own power base to the table. If you stay in the race and show you have serious support within the party, then there's going to be pressure to pick you.
Also, to a certain degree, there's cabinet posts up for grabs as well.
Next, the fact is that some people run because they want their message to be heard, which is why Al Sharpton or Jesse Jackson runs (although Jackson always had a better real shot at it). Quitting early takes away your pulpit and place in whatever debates are coming up. Sharpton may have only single digit support, but that translates into one whole podium on TV.
Finally, it's quite possible that the remaining candidates just don't want Kerry to win, for whatever reason. They can't stop him, but they certainly aren't going to bow out gracefully. Of course, Kerry is not all that polarizing, really, but who knows what is running through the heads of some candidates? Of course, eventually the Democratic Party will step in, just like you'd expect, and pretty much tell the losers in that case to line up or face a lot less support for future campaigns.