The political theory is that the Democrats are a "Big Tent" party, and for Democrats to be elected to Congress in states and districts with less liberal populations, you will naturally end up with less liberal Democrats. And that not only makes sense, it should allow you to build a larger, more diverse caucus. But here's the thing. That -D at the end of your name has to still mean something in the end. If a representative is so conservative that he or she is effectively part of the opposition, then you are no better off than if he or she has a -R after their name.
A working majority is a valuable opportunity for legislators to implement their agenda, and the agenda of their party (which should be very close to the same). And all things being equal, a larger majority is better. But, in the case of the Blue Dog Democrats, all things are most certainly NOT equal. It's not even that they often vote against their caucus. One of the advantages of a larger majority is that it can absorb a more significant number of defections. It is a longstanding political tactic to whip enough votes to pass a bill, and then select those lawmakers for whom the vote would be politically problematic and let them know they can vote no and the bill will still pass.
No, the problem with Blue Dogs is not when they don't support a bill, but rather when they actively undermine the caucus and it's agenda. When they form alliances with the opposition to water down important legislation, or stand on the Capitol steps in front of the microphones and repeat Republican lies and talking points. When they siphon off Democratic money and support and then act in ways destructive to the goals and aspirations of their fellows and their leadership.
According to Nate Silver's Projections, the expected House Democratic minority of 203 seats will include just 27 Blue Dogs, representing 13% of the caucus, down from the current 21%. Now a Republican Majority in a time when bigotry, fear and ignorance is ascendant is by no means to be welcomed, but in general, a smaller but more cohesive and disciplined Democratic caucus would be more desirable than the chaotic and unfocused group we saw in the 111th Congress.
We probably will never be completely rid of the Blue Dogs, but as the American political process evolves towards a more parliamentary system where the parties are unified around both an ideology and an agenda, they will lose their bite and will find it less beneficial to bark. The goal isn't personal aggrandizement nor is it necessarily even deal making. The goal is to accomplish something, to define an agenda and find a way to implement it. And the time for dissension within the ranks of the party is quickly becoming a thing of the past.