Ask Mr. Smarty Plants

It is true that plants native to arid lands employ a variety of methods to reduce the amount of water used, including deeply-growing roots, fleshy water-storage organs (roots, bulbs, stems, leaves, etc.), reduced leaves and waxy epidermal coatings.  Another strategy for water conservation used by many plant species native to areas of low rainfall is CAM photosynthesis.  

Arid country plants do not conserve water so much by sequestering less water in their tissues -- to the contrary many of them actually store more water than species native to areas of higher rainfall.  Rather, these species reduce the amount of groundwater removed from the soil and passed to the atmosphere through the process of evapotranspiration.  CAM plants accomplish this by enabling their stomata to remain closed during the day when evapotranspiration rates are higher and carry out their photosythetic process at night when evapotranspiration rates are lower.

So the CAM process itself is not so important to water-use reduction as what the process allows those species employing it to do.  That is, it allows them to carry out photosynthesis at night when evapotranspiration is reduced.  With less than 10% of the earth's plant species employing the CAM carbon fixation process, those plants that do, use less water than those using the more typical C3 and C4 carbon fixation pathways.

Whatever water-saving strategies are employed, the important fact is that plants native to arid lands are well-adapted to their environment and the reduced availability of water there.  For that reason, the method of water savings is somewhat moot.  Native plants' adaptation to low water availability is in large part why we strongly encourage their use in landscapes within their native ranges.