You''l find many folks on here rarely use a secondary for regular beers and just primary for at least three weeks. For a pro/con analysis, see the "This vs. That" sticky in the Beginner forum.
Whether you secondary or not, the longer it sits in a fermenter (up to a certain point, the better). It lets the beer clear and the yeast have time to consume some of their earlier by-products.
Many people use a secondary to minimize contact with dead yeast, which can impart off-flavors (autoliysis). However, many people now believe it takes several months for this to happen. Longer time in primary allows more yeast to clean up the by-products, as mentioned earlier.
As you've probably guessed, I attend the primary only school.