What's the logical answer to "is a vote for a 3rd party candidate in the presidential election a wasted vote?

Again, I'm seeking a logical, intelligent answer. I'm 48 years old and for the first time in my voting life I'm not willing to vote for either the Democrat or Republican candidate. I've always believed that a vote for a third party candidate was a wasted vote, but I'm wondering whether anyone can provide a logical, articulate argument to convince me otherwise. I hate to choose the lesser of two evils, which is what I feel I would be doing if I vote for either McCain or Obama.