Edwards is important to the current campaign because he's a flamboyant and highly effective campaigner, and Kerry is already getting dumped on for being boring. This campaign will be a hard one to win, and Kerry needs to choose a running mate who will attract votes, not one who only appeals to a few party insiders whose votes are already in the bag.
The real reason that choosing Edwards is vital is that if Kerry does win, and is reelected in 2008, whoever his VP is will be the Democratic candidate in 2012. If he didn't pick Edwards, he was almost sure to pick Gephardt, which would have been ceding the 2012 election to the Republicans without a fight even if it didn't cost him the current election. 4 years of Bush have done so much damage to this country that it will take more than 8 years with a Democrat in the White House to fix it. (I figure 8 years of apologizing to the rest of the world might be long enough for them to talk to us again, and the Democrat after Kerry might actually be able to do something in the international arena.) We must have a VP this year who can win in 2012, or else it's time to get serious about moving to New Zealand.