I currently drive an SUV, which I will keep for towing my trailer, bigger shopping trips, etc. Right now I drive about 10,000 miles a year at 18mpg; once I get the smart that should change to 8,000 miles at 50mpg and 2,000 miles at 18mpg. In addition, the reduced wear and tear on the SUV means it should last at least another 10 years (and I intend to keep it until it falls apart).
If you compare the alternative (i.e. buying a new SUV in a couple of years), the overall savings in energy consumption over the next decade (including manufacturing, operating and recycling the vehicles) represents a worthwhile net gain.