In the past the water softener sales pitch had included benefits to your plumbing system. The idea behind increasing the life of your water heater is that you would reduce the sediment that builds up inside of the tank. Sediment can have a negative impact on your water heater efficiency and overall life of the tank, so it's a good idea to flush your water heater every 6-12 months.
Anode Rod & Glass liner
If you have a water softener you should replace your anode rod ever 3 years. Most plumbers know this and will make the recommendation if they install a water heater. The salt from the softener will eat away at the anode rod and will usually disintegrate within a few years.
Newer water heaters have a glass lining on the inside of the tank. This glass liner has hundreds of air bubbles that create pockets that are susceptible to the sodium from the water softener. Much like the deterioration of an Anode rod, the glass liner of the water heater also seems to break down.