For the record, I'm not big on the unity idea. Clinton is a plausible president but she'll alienate as many voters as she brings, and most of the ones she brings will end up voting for Obama anyhow. She's an obstacle to the whole "break with the past" motif. Noah Millman's article lays the case out compellingly:
The VP pick should, ideally, accomplish several, if not all, of the following objectives: reinforce the campaign narrative; strengthen the image of the nominee; bring in a valuable organization for GOTV or fundraising; win over an otherwise difficult-to-win constituency; retain a trusted counsellor; anoint an heir apparent.
Obama needs a plausible president who can speak to the cares and concerns of ordinary Americans. Somebody people know. Somebody who complements Obama's story nicely, who has a similar message. That person is John Edwards. And I've not exactly been a fan of his historically, but it makes too much sense to me not to pick him. I suppose there's the lack of executive experience--as if McCain can capitalize on that.A Clinton pick actively undermines the campaign narrative; weakens the image of the nominee; does not provide him with a trusted counsellor; and does not anoint an heir apparent. The only reasons to add her would be: for the Clinton “network” and organization, and to win over an otherwise difficult-to-win constituency. I don’t think Obama needs the network or the organization. That leaves one reason.
Obama had trouble, in differing degrees, with four constituencies who were loyal to Clinton in the primaries: women, older voters, Appalachian whites, and Hispanics. I don’t think Clinton helps materially with Hispanics. She could very well help with Appalachian whites, but I don’t think she’s the only pick who could do so. Ditto with older voters. And I remain unconvinced that Obama has a “problem” with women voters in a general election against McCain.