quote; Sometimes, I feel like the customer is not buying into the idea of the expansion tank and thinks I'm just trying to sell them on things they don't need.
That is EXACTLY what you are doing. An expansion tank has absolutely NO function unless the heater is in a closed system, and even then, if the house was an a well the storage tank would act as an expansion tank.
quote; I usually tell them that water heaters are not built like they were in the past and that also the city water supply can be tougher on water heaters than in the past.
That is either trying to dazzle them with your brilliance, or baffle them with B.S., but either way it has nothing to do with the need for an expansion tank.