They are probably also responding to the increasing water rates and public education regarding the water issue. You are correct in suggesting that a rock landscape is hotter than a landscape with plants. You are also correct in your statement that rock landscapes are not considered xeriscapes by many horticulturists and proponents of xeriscape. Rock landscapes do reduce landscape water use, but the savings in water is not as great as some people think because water used in evaporative coolers is increased. Rocks can be an important part of a true xeriscape which reduces water use, but not at the expense of the beautiful landscape.
Rocks are an excellent mulch material to use around plants native to hot, dry environments.
Plants native to cooler, moister parts of the world may have difficulty surviving the heat that is common in rock mulches in the summer in New Mexico. Plants native to cooler environments do better with an organic mulch. For those plants that will tolerate the heat of rock mulch, and there are a very large number of such plants, the rocks reduce weed competition and evaporation of water. Beneath the mulch the temperatures are also lower. If the rock mulch is to be effective in reducing weed growth, it should be three to four inches thick.