How Valuable is an Old Email List?

Kari Chisholm and Taegan Goddard both have pointed to a report in The Hill that credits John Kerry with a big advatnage because of his 3 million name email list.  Kerry's 2004 Finance Chair boasts:

“There is a zero marginal cost to communicate with them,” Randlett said of voters in Kerry’s database. “He can wait much longer [to decide to run for president] and suffer a much lower degradation of his base."

I agree the marginal cost is low (though not zero), but the claim about degradation of the base is pure baloney.  Owning a mailing list of past supporters does not guarantee that they will be future supporters.  Certainly, it's an asset to have, but it would only permit a delayed entry if a large portion of those people could be assured to be with him in the next primary.

How many of those names were there before he was the Democrat nominee?  The names that he picked up during the primary might be considered a base, but the general election names only preferred him over Bush ... not necessarily over any other Democrat.

For example, of those 3 million, how many are also on Howard Dean's list?  Or John Edwards'?

Email lists from previous campaigns are valuable, but they'll only take you so far, especially in presidential politics where voters tend to be more fickle.