Depth of field drastically increases the photorealism of a render. It gives a very dramatic effect to your images. A word of caution though: You should not make it too heavy, as it can also ruin much of the subject involved. I'm always a bit in doubt when using depth of field. The reason being sometimes you want to emphasize a certain part of your design, but at the same time, you want to show more of that glorious geometry in the same image. At the end of the day, it's all a matter of taste.
I recommend playing around a bit with DoF. It's very simple to add, and can potentially add huge dramatic impact to your design, as well as increasing the photorealism of your rendered image.