I think Luntz is a linguistic genius, and having seen him up close at a few gatherings, he doesn't seem like a scumbag. He's really good at what he does.
He understands the connotative effects of language as well as anyone in the world, and his book, Words that Work, is a must-read for anyone who wants to understand political messaging (cough, cough, that's us folks).
His point about liberty is a good one, and I agree to some extent that his basic point - freedom is personal, liberty is ideological - might have some bearing for some of the electorate. Maybe most of it.
One thing I think Luntz misses, however, is that a candidate who talks about "liberty" is sending a signal to people who are of a libertarian/constitutionalist worldview (in a Republican primary, at least 10-20%), and that is a "codeword" that this might be someone who thinks like they themselves do.
It is an interesting point and one, frankly, that I haven't considered. Myself, I have always preferred to use liberty, but for some reason, now that I think about it, I realize that I use "freedom" more when around mainstream Republicans. Hmmm... never really thought about this.
Interesting.