There weren't any obvious "bad choices"... Kerry would get a bump no matter what... but out of all the candidates, Edwards is by far the easiest to negatively define...
Hillary would have been the easiest to negatively define, but she would have eclipsed Kerry. Edwards will be easy to attack for a host of reasons. I am glad that Kerry didn't select a Sam Nunn who would have presesnted some real problems in terms of competence on national security issues.