Container gardens with cacti and succulents will look great and benefit from a layer of small rocks on the top of the soil. The rocks will insulate the potting soil. In hot, dry and windy weather, moisture will quickly evaporate from the soil, and this layer of rocks will keep moisture inside the pot and cut down on the watering you'll need to do.
But remember that if you plant your container very densely, the layer of rocks won't be as necessary because the plants and the roots below will insulate the soil.
What do you think? Don't you think this planting looks great with the rocks?