For me, this is easy - it's all about the Benjamins...
It much easier to keep one car with fuel in it to run the kids around and for my wife to get to work than it is to keep 2 with fuel. Especially with gasoline being above $4. Running two cars is just to much for my budget to handle, but it can handle 2 cars occasionally. It also cheaper to get on my bike and ride to work than pay for a gym membership. And how much time do you have to spend at the gym to make that work for you? It lots of little things, but mostly it is for the green in my wallet.
I drive on the weekends, but that is mainly because I'm taking a few kids along or buying items to large for my trailer.