Depends.
It depends on what you are brewing and how long you are going to leave it. It also depends on whether you are making additions or not.
If I brewed a really big beer, something over an OG of 1.100, I might want to age it for a long time and then a secondary might be good for it. If I was going to age a beer on oak I'd want it off the yeast so the oak flavor wouldn't be absorbed by the yeast. I've found that leaving a beer in the primary and adding fruit isn't the greatest idea either.