Except it wasn't just a list of voters. It also included "client scores". That means the Sanders campaign had access to modelling information regarding the Clinton campaign's list. It is pretty valuable knowing how the other campaign values and/or targets specific voters and that is something that obviously can't be found from public info.
Re:The DNC data breach
by toomuchtodo on Friday December 18, 2015 @06:21PM
(#10761661)
> That means the Sanders campaign had access to modelling information regarding the Clinton campaign's list.
And also means that Clinton's campaign had access to the Sanders' campaign data.
Even then, you only have a snapshot, because the states typically don't keep historical data. What this means is that your dataset won't be as good as someone who's been collecting this data for years, and thereby knows things you won't like where someone used to live, how often they voted there, who recently dropped off the registered voter rolls, etc.
In this case, even this data wouldn't be enough, because the Sanders team had made likely hundreds of thousands of contacts with voters, and recorded what issues they cared about and who they planned to vote for. This data, which they personally collected, is now inaccessible to them.
edit: expounded