The problem with stupid people is that it takes an enormous amount of time and effort to refute their stupidity. Therefore, most people don't bother - so the stupidity just keeps getting repeated with no response from any smart people. And therefore the semismart people believe it.
This was originally posted as part of a comment at Stonekettle Station. Then Chris Gerrib decided he liked it enough to quote it on his own blog. So it made sense, to me at least, that I should quote myself on my own blog as well. :)