[trashtester]

The main problem with your argument, is that for many use cases, the real voltage from a battery is not identical to the nominal voltage. As the current through the battery increases, energy loss due to the internal resistance of the battery will cause the external voltage to go down.

This means that actual Wh will depend on how many amperes the battery has to deliver. When the load is very low, this hardly matters, but when you need to power a high wattage engine/appliance with a small battery, this can be quite significant.