It doesn't matter, the phrase means what it means and when someone uses it, we are communicating. But I think that the issue is not one of grammar but of meanin

It doesn't matter, the phrase means what it means and when someone uses it, we are communicating. But I think that the issue is not one of grammar but of meaning. Saying you could care less is not grammatically incorrect, it just doesn't seem to mean what the sayer is usually trying to get across. My guess is that time has turned "I couldn't care less" to "I could care less" just by virtue of it being easier to say.