Look, Lieberman is not the most conservative Democrat, Ben Nelson is:
http://www.nationaljournal.com/voteratings/sen/lib_cons.htm?o1=lib_composite&o2=descOn many issues, Lieberman is indeed in the liberal wing of the Democratic Party:
http://www.electoral-vote.com/evp2008/Senate/senator_ratings-2005.htmlHe's an asshole for backing McCain and a nut when it comes to Iraq, but other than that he is quite tolerable.
We need him because the best tool the Republicans have to block Obama's plans is the filibuster. Not only can Lieberman be a reliable vote on issues such as health care, taxes, and appointments, he is also an important conduit to the Republicans. He has influence with them that no other Democrat has.
I don't like Lieberman at all, but I think the place to punish him is at the voting booth, not in the Senate. We need his vote and support, and Reid can always take away his leadership role at a later time should he fail to cooperate on key issues.