Third Parties and Independents

I got my voter information pamphlet in the mail today in preparation for election day and it got me thinking about the third party and independent candidates. I have also had comments on my poll regarding the fact that I have only listed Democrats and Republicans as options.

My stance on these candidates and parties is that they are a good part of a healthy political system. Considering how rarely they ever get voted into office I wonder if they serve any more functional purpose than to promote fringe ideas which might later be adopted by one of the major parties. The keyword there was functional. Does anyone see any other tangible benefit that comes from these types of candidates and parties in our system?