The Democrats' current majority contains a number of seats that are vulnerable even with incumbents (TX-22, FL-16, GA-12), and a number of other seats which will be vulnerable when the incumbent retires (TX-07, UT-02). The more seats they can grab that will be safe with incumbents, the better position they are in to hold on to the House in 2010, 2012, etc.
GA-12 in its current incarnation is only marginally Republican-leaning, and there are lots of conservative Dems there who could fill in for Barrow, particularly given the relatively weak Republican bench there. It's GA-08 that's the real problem - without Marshall, this district is absolutely lost.
