Blue Dogs are ok for the Democrats if you have an opposing party that's willing to compromise. But since the modern Republican Party does not compromise, the Blue Dogs are useful to Republicans so that they can say, "See, even some Democrats didn't vote for Obama's legislation!"
Tell me, which other Republican-held seats,
above and beyond those that the Democrats are already targeting, should they target to make up the difference after ousting the Blue Dogs? As far as I'm concerned, it is
infinitely more important to have 218 Democrats in the House than it is to have any sort of "ideological purity." There are currently 25 House Democrats in the Blue Dog Coalition. Without them, it's 25 more seats the Democrats need for a majority.
