Dogs could also be good as an early warning system for the walkers. And this brings up another thing, whatever happened to the Horses at the Greene farm? Did they just leave them to be eaten? Personally, I would think keeping them around would be a good idea. They're a quieter and easier maintenance alternative method of transportation.
Which leads me into another thing, the amount of time we see them driving cars. I know there are a lot of cars full of gas around, but it's going to be a limited supply if no one is actively bringing more in. I would think pretty soon they would want to start being more careful, and limit how much they drive around. Not to mention that I would think the noise of a car engine would attract Walkers.