If that was their purpose then you think they'd rig the poll to put Roberts and Brownback slightly ahead. That way they'd encourage the Dems and the Independents and the moderate Republicans that they need to get out to the polls.
The Democrats that I know here in Kansas don't really think that they are going to win those three races. They assume that the wandering Rs will return on election day. They have seen some recent polls which have shown the movement towards Roberts and Brownback which fuels their fears. This poll encourages them that maybe, just maybe, the tide isn't coming in after all.
IMO, if the polls are being selectively reported, a poll showing a slight lead for Roberts and Brownback would be interpreted by the Democrats as further evidence that the tide they fear is rolling in. I think that your suggestion would decrease, not increase, Democrat turnout.