Ideologically you are 100% right.
This is politics, my friend. The Democrats will do what's right for them as far as pushing their message forward. The message of "revenge before national security" isn't going to resonate despite how you, I, and perhaps even the Daily Kos would like to reframe the debate.
The debate is not over whether Lieberman deserves the spot, the left-wing blogosphere does not depict how the rest of the world will see it. If Lieberman gets removed, it will be perceived as revenge.
Not just as an inevitable conclusion to an already too long drawn out divorce?