The Libertarian is voting Libertarian. I can see no reason for a Libertarian to vote for a Democrat.
IANAL-I am not a libertarian.
I find doctrinaire libertarianism a bit whacky, but if I was a dedicated Libertarian, I would vote third party. If you really hold those beliefs, it's not a matter of picking the lesser of evils.
Same thing with a communist, no I am not comparing the two. But if you genuinely believe in communism, voting for democratic style socialism would be repulsive as well. I disagree with the philosophies, but I do get a bit annoyed when folks here try to suggest that Libertarians go and vote for Chris Shays or Lincoln Chafee and they are horrible human beings if they don't.