A thoughtful post on the often misused phrase, “I could care less,” by the estimable Grammar Girl mines the many reasons mangled phrases gain traction. Most startling is the rising occurrence of the spoken misuse compared to the correct version, “I couldn’t care less.” You can see it rise graphically in this Google Ngram.
It’s easier to get away with misuses in speech because people tend to hear what they expect to hear. However, if your listener picks up your gaffe, or if you write it incorrectly, you undermine your own message. If you “could” actually care less, it means you still care a little bit, probably not what you meant. And if you assume people know what you mean anyway, you’re forgetting about the army of language curmudgeons that lurk under every pair of scowling eyebrows. Oh, they get irritated. And you get discredited.
Also read the Comments section, where there are many thoughtful shares, including this video, a terrific David Mitchell rant shared by “Will.”