Why does it irritate me so? Because it makes no sense. It is often used in place of the original phrase (penned by the British) of "I couldn't care less." This statement works. It makes sense. When I say it I mean exactly that: I care so little about [insert particular comment/rant/article/person here] that I actually could not care less. That is to say, on my care-o-meter I am officially at or below zero.
So where did the illogical derivative come from? Well, leave it up to us good old Americans to take something that actually makes sense, and turn it around so that it doesn't. But this raises the question: What happened to the negative? Perhaps some sarcastic punk wanted to really emphasize his/her apathy when he/she said to his/her equally stoned mate "Dude/ette, like I could care less." Now, that makes sense. The person is, in a sarcastic manner, emphasizing that he/she is totally apathetic. Sounds like a reasonable language permutation to me.
In the ensuing years, though, this phrase was repeated again and again, by ignoramuses (my own personal bias there) until it actually came to resemble a proper use of language. Now, there are plenty of places discussing this improper use of the phrase. I link because I care. The point that some (not all) of these references clearly make is that, in the spoken language, vocal inflection can lend meaning to the phrase "I could care less." Delivery can emphasize the sarcasm the speaker may have intended.
However, the written word cannot. I have previously evangelized that all written communication on the internet should have 'sarcastic green', a vile color that is used to warn the unsuspecting reader that the offensively colored words are meant to be sarcastic. Think of the confusion this would clear up. Lamentably, this idea has not gained widespread acceptance, and we, the readers, are forced to infer (often from barely literate writers) what passes as sarcasm. In the end, failure ensues, ideas are miscommunicated and a visual diarrhea of smileys is used in an effort to smooth things over.
My personal bias (again) is simply this: Most people who write and say that they "could care less" are simply stupid. They aren't trying to be witty or sarcastic. They are ignorant, and haven't given thought to just how silly it sounds/reads when they say/write "I could care less." Perhaps I should give them the benefit of the doubt.
Unfortunately, most bloggers haven't given me a good reason to. In the end, as evidence by the fact that I wrote this article, I actually could care less.