She is ancient, and I do mean ancient history.
She would add the memory of "Clinton's economy" and "the peace and prosperity of the Clinton years" to the ticket. I understand you would never go for that, and neither would I, but many would. Accept it or not, the Clintons are still popular. It wouldn't matter if she doesn't add anything of substance to the ticket, because the "promise" of another Clinton in the White House might be enough.
It could work for them, if we don't have answers to "Clinton's economy" ready.