Ask five homebrewers, and you'll get five different answers.
For me, I rarely use a secondary unless I'm going to be aging it for a while before kegging, trying to get a super clear beer, or adding fruit or wood. However, I rarely keep beers in primary less than 3-weeks. It gives the chance for the yeast to do their duty and let it all settle real nicely.
I look at it this way (and again, you will get different answers from different folks). Every time I rack my beer, I run the risk of introducing oxygen or an infection to the beer. The less I move it, the happier it is.