I agree with Juan.
Of course, you shouldn't do it sneeringly or patronisingly, but I too don't understand how it's really different from any other kind of knowledge that people may not know correctly. Isn't it better for somebody to kindly mention it rather than to leave you in your ignorance, applying for jobs, sending off important communications and never having a clue that people might be laughing at you behind your back?
The problem is that, if a common misuse gains enough ground and goes unchallenged for long enough, you will find that the person using it correctly ends up firstly being mocked for showing off and then, before too long, ridiculed by the uninformed majority for being supposedly wrong, when they are in fact correct (and I'm not just talking about Dan Quayle).
Using the example given earlier, can you imagine people regularly mocking you for stating that Washington DC is the US capital, purely because enough people mistakenly believe it to be New York?
Another one that absolutely baffles me is 'nucular'.
"If something isn't old and has just been made, how would you describe it?"
"New."
"How would you complete the phrase 'Let's make sure that the coast is....' ?"
"Clear."
"So how would you describe electricity that's produced by the fission of uranium and plutonium?"
"Nucular."
Aaaaaaggghhh!!! You can obviously say it, so why could you possibly think that it's pronounced that way?!