Branduil is right. The unicorn argument is basically people high fiving each other and going "yeah right, you tell 'em". Its purpose is not to actually convince, but to ridicule and make light of the argument at hand. No one is going to "Wow, you're right, my belief in God is the same as belief in unicorns, I guess I shouldadd unicorns to the list of things I believe inrenounce my religion".
Everyone who uses the argument knows this.
Do you think anyone has any chance of convincing ANYONE to change their religious beliefs because of an anonymous messageboard posting?