Biden or Gore would do even worse. Hillary has the advantage that even people who may not like her personally or who dislike her policies will still vote for her, just want to see the first female President (i.e. the same advantaged Obama had among the starry-eyed who wanted to see the first black President). She also has the full support of the Democratic National Committee and the rest of the party apparatus, they aren't going to change gears and suddenly support Biden. That's why the only challenger to Hillary in the double digits is Bernie Sanders, a fringe candidate who will never get much beyond 10% support, mostly coming from college campus radicals.
Having said that, the GOP's frontrunners right now aren't all that encouraging either. Topping the polls are Jeb Bush, who may as well be a Democrat, and the clownish Donald Trump who seems to say things just to keep his name in the headlines. Conservatives may very well stay home or do a third party protest vote of either of these is the nominee.
Gore and Biden running helps a lot. They can beat her up in ways that the media won’t do. They can hurt her among the Independents. With Sanders on her left, Webb on her right, and the other two in the race she will have to spend more, work more, run greater risks of showing how nasty she is, and end up more damaged than if she were simply coronated by her party.