Rooting for U. S. Steel

I had a good weekend. The NY Yankees clinched the American League East, and the NY Giants shut out Tampa Bay.

I've been a Yankee fan since childhood, but even as my interest has waxed and waned over the years, I am happy when they do well.

It was said during the 1950s that "rooting for the Yankees is like rooting for U. S. Steel." Which got me wondering why people choose to root for the underdog.

There are some tentative explanations on the web:

http://answers.yahoo.com/question/index?qid=20071012200500AAjY5rH and

http://psychcentral.com/news/2007/12/24/why-do-we-root-for-the-underdog/1699.html

But I am wondering how philosophers of sport think about the question . . . why do we root for the underdog? Should we root for the underdog?