I know a lot of people say "I vote for the man and not the party." Well I'm a Democrat and I vote for the party primarily. The reason I do is that the individual who is elected by the party is beholden to the platform of the party. Given that you can never tell what some of these politicians believe themselves, (George Bush Senior turned out to be pro-choice for example) I want to vote for the party that reasonably stands for what I stand for. The only 3rd party candidates I see out there right now will probably be Tea Party types. None of them would ever get my vote.
Also give our current system a third party President would accomplish only one thing. He'd unite Democrats and Republicans to see that nothing of substance happened in the four years he was in office.